Izuku grunted as he pulled on the rope tied to the trashed refrigerator. Just like what All Might had said a couple days ago, training to become the next vessel for One for All was tough. He had to clean up all of Dagobah Beach, since it had been covered in trash that drifted in from the ocean's currents, which people took advantage of to illegally dump their own garbage since no one could tell the difference. So far, it has been torturous, even with the enhanced strength he had gotten.

"I don't want to imagine how this would've been like if I didn't get stronger recently," he muttered to himself, as he finally moved the refrigerator into the truck that All Might had gotten to carry the garbage to the proper trash facilities.

All Might was simply amazing for coming up with such a training regiment within a short period of time. The schedule he had given him factored in his school work, and took advantage of every space of time he had available to train his body. Not to mention that it allowed him time to work on solving the mystery of his kidnapping.

"How you're holding up, Midoriya?" All Might asked, as he sat on top of one of a broken washing machine. "I see that you've gotten quite the trash heap on that truck! That's pretty good, I didn't think you'd be that fast with gathering all that garbage."

"I'm doing fine, All Might," Izuku replied, wiping the sweat off his brow. "I think I'm gonna have to call it a day soon, though, since I have to get to class soon."

"All right, but remember to head back here as soon as you can! We have to use as much time as we can to help refine your body!"

Izuku nodded, saying goodbye quick, before heading home to wash up and get changed. Couldn't smell like sweat and sand at school, after all.

The day before yesterday was a really busy day for him. Getting smacked by that sludge guy, working a full time job at the Institute, saving that explosive kid who got held hostage by the same sludge guy and got turned into a Smash? And then going back to work like nothing happened the day after? He was worn out.

At least until he heard a ding. A tuft of hair sprung up in a cowlick, as a giant grin spread across his face immediately, and he rushed over to the machine. It was a large, metal cube with a sliding door, and next to the door was a microwave sort of contraption. He opened the microwave, and pulled out the bottle that was inside. It was red, with a bird-like creature taking its shape.

"Oh, so we got the Phoenix Fullbottle?" he said, marveling at the bottle. "Makes sense, considering that explosive kid, huh, Misora?"

The door to the machine opened, and a short girl with short black hair with a ponytail sprouting from the top of her head walked out. She squinted at the man for a bit, before muttering, "Tired... Sleepy... Want my money, Sento..."

She walked off slowly, before plopping herself onto a nearby couch to sleep. The man, Sento, sighed and shook his head. "I'll give it to you when I get my next paycheck tomorrow," he said, as he headed towards a work bench with his belt on it. "Let's see... how about Gatling and Phoenix?" he muttered, inserting the Phoenix Fullbottle and a gray one that was shaped like a gatling gun barrel into the belt.

"Phoenix! Gatling!"

"Hm, didn't work... how about Phoenix and Jet?" he said, taking out the Gatling Fullbottle and replacing it with a blue one shaped like an airplane.

"Phoenix! Jet!"

"No, not it either..."

"Hey Sento!" a voice called out from the top of the stairs. Sento turned around from his work bench, and saw Misora's father, Souichi Isurugi, coming down. "Turn on the news, they're talking about Build."

"Really!?" Sento said, as he grabbed the remote and turned on the TV. Sure enough, there he was, a clip of his fight against the explosive kid's Smash form ("Hm... I guess he could be called the "Explode Smash, all things considered," he thought). At the bottom of the screen was the headline, "Who is Kamen Rider Build?"

"Oooooh, that's some good footage they got!" Sento said, a manic grin spreading on his face. At least until the newscaster opened his mouth, questioning Kamen Rider Build's integrity, and if he was the one responsible for turning the explosive kid into a monster. For the next few minutes, the airtime was full of interviews from pro heroes who also questioned his objectives and bystanders who were just amazed that the Kamen Rider even existed.

"How rude! Kamen Rider Build would never turn people into Smashes like that!" he said, aghast at the accusations that were being thrown at him, though the bravado did give way to uncertainty. "Wish I knew where the people who do are, though."

"Hey, don't sweat it, Sento!" Souichi said, giving him a hard pat on the back. "You're doing a great job so far, so be proud of that!"

Sento grinned. It had been a few months since Souichi found and took him in, helping him with housing and giving him the resources he needed to become Build. Sure, there have been some rough bumps (mostly the first time he fought as Build, since he didn't know what he was doing, and that Fang Smash really did a number on him, though at least he got that Lion Fullbottle), but he got the hang of it after a while. He checked the clock, and jumped up from his chair.

"Oh man, I'm gonna be late! I gotta go, Owner, see you later!" he said, grabbing his coat and belt. He climbed up the stairs, and out through the secret mini-fridge entrance. Once outside, he inserted the Lion Fullbottle into his cellphone, and watched it transform into a motorcycle.

"Welp, today's a new day, hope nothing goes wrong for me."

Everything had been going wrong for Bakugo for the last two days.

He got attacked not by one, but two villains, and some asshole vigilante who's gear was way too advanced and strange for his taste. And not only that, but his Quirk was now gone. Sure, that vigilante spouted something about it being temporary and only lasting two to three days, but just one day was just one too many for him. Not to mention that he was forced to skip class by his parents who coddled him over the whole situation, which ruined his perfect attendance record.

As he walked into his classroom, his worthless classmates stopped whatever they had been doing to just stare at him. He scowled, and made it over to his desk, trying to ignore the weird looks they were all giving him.

"Hey, Bakugo, think fast!" one of them finally said, tossing a small rubber ball right into his face. Bakugo tried to blast it away with one of his explosions, but nothing happened, and it collided smack dab on his cheek, knocking him out of his chair.

"Holy crap, it's true, he's lost his Quirk."

"Wow, couldn't have happened to a better guy."

"Looks like the king of the school's been dethroned."

Fuck, they were all looking down on him now. He growled, and slammed his hands onto his desk. "Shut the fuck up! It's only temporary, so don't get used to it!"

His classmates chuckled, as if they weren't intimidated by him anymore.

"Boy, that's tough talk from someone who's as useless as Midoriya right now."

"Pft, Bakugo's pretty much nothing without his Quirk."

"Always hated his guts anyways."

"Hey Kacchan."

Bakugo scowled and turned to face Deku. That miserable bastard stared at him with pity. It was unbearable. To think he'd be reduced to his level.

"Go the fuck away, Deku, I don't need your pity," Bakugo muttered, looking away. "Unlike you, my Quirklessness isn't permanent, so back off."

"Bakugo I-"

"Alright, everyone settle down," their teacher said, finally entering the room. "Now, I know that a few of you," he said, glancing over to Bakugo and Deku, "had a pretty crazy couple days, but for now, I just want you all to focus on class, is that clear?"

The class muttered a weak "Yes, sir," and it wasn't before long that the lesson started for the day. In between classes Bakugo had to deal with the jeers and taunts of his worthless classmates as he struggled to force an explosion to come out of his palms. It wasn't until school was over and he was outside that his troubles really started that day.

"Hey, Bakugo," a cold, hard, deep voice said. Bakugo turned around to face Ishishishi, a guy two years his junior whose mutant-type Quirk made him a giant rock man with rocky mane-like hair. "Heard your Quirk's gone."

"Yea, and? It'll be back soon, so what about it-"

Ishishishi picked Bakugo up by the scruff of his shirt, and grinned. "That means that you're no longer the big man on campus, and that I can take your place."

"Oh come off it, Ishishishi, we all know that when the sun's gone, you freeze up and turn into a statue," Bakugo growled, not intimidated by Ishishishi in the slightest. "So let go, and I promise you that I won't break-your stony ass into a thousand pebbles when my Quirk comes back-"

Ishishishi slammed Bakugo against the wall that surrounded their school, not amused by Bakugo's defiance. "I guess before you do that, I'm gonna have to send you to the hospital. You don't get it, but without your Quirk, you're nothing."

"Leave him alone!"

Bakugo and Ishishishi turned their heads, and saw Deku looking at them. Bakugo's scowl grew larger, as he muttered, "I told you, I didn't need your help before, and I don't need it now! Now buzz off, and let me handle my own business!"

"Seriously? You?" Ishishishi laughed, shaking his head. "At least Bakugo's Quirklessness isn't forever. You? Just stay out it, and hope that I don't snap you in two."

Ishishishi refocused his attention back onto Bakugo, who just gave him the most defiant scowl he could muster, while he readied his rocky fist for a punch. When he got his Quirk back, Ishishishi's gonna pay dearly.

Their staredown got interrupted once again, but this time, Deku barreled into Ishishishi, knocking the stone giant boy to the ground. Before anyone could really process what was going on, Deku grabbed Bakugo's wrist, and ran off with him in tow, everyone too stupefied to understand what just happened.

"You bastard!" Bakugo scowled, once Ishishishi was out of view. He wrenched his hand out of Izuku's grasp, and glared at him. "I had that under control, So I didn't need your worthless help!"

Izuku froze. He never meant to slight Bakugo's pride like that, but he just couldn't let what was happening slide by. It wasn't fair. No one deserved to be Quirkless, permanent or temporary. In the moment, he related a lot to Bakugo's plight, since it's been like that all his life.

"I know, it's just... I can relate with what you're going through," Izuku said, looking away.

"Shut up."

"Kacchan, I know you can bounce back from this, but it doesn't hurt to have someone support you in the meantime. I know for one thing that I would've liked a friend to help, and-"

"I said shut up!"

Bakugo shoved Izuku away, scowling. "I don't need anyone's pity! I don't need yours, my parents', anyone's! So I'm Quirkless for a few days, big fucking whoop! I'll plow through this, no sweat! This is nothing but a small road bump, and I'm not gonna let this stop me!"

Izuku looked at him. It was true, this was just a temporary setback for Bakugo. That was what he admired about him, that endless drive to become the best, even if the odds were against him like this. He took a deep breath, and smiled.

"Alright then, I'll let you be, but I want you to know this, Kacchan," Izuku said, as he steadied himself. "I'm still aiming to be a hero, and like you, I'm not gonna let anything stop me anymore! Just you watch!"

Bakugo stared at him, and scoffed. "Fine, whatever. You can keep dreaming like the idiot you are. Your stupid burst in strength isn't gonna carry you as a hero."

He turned around, and walked off. Izuku sighed, and started to head off in the opposite direction. He had to get back to his training, after all. Nothing was gonna stop him from becoming a hero.

By the time Izuku had returned to Dagobah Beach, he heard some noise coming from the garbage heaps. He headed towards what was causing the noise, being sure to be as stealthy as he could to avoid detection. When he peered around the corner, he gasped. He saw another creature, one that looked similar to how Bakugo had when he had been transformed by the gas bullet. This one looked more akin to a giant mechanical hand with two legs crushing a rock, with its arms looking like the pinkie and thumb, while the remaining three wrapped around its "rock" torso. The middle finger had three holes on the fingernail, making it seems like that was where its face was.

"The guy who transformed Bakugo must've done this," he thought, as he peered at the creature, who wandered aimlessly around the trashed beach, sometimes smashing a refrigerator or two with its giant finger-like arms. "Doesn't seem like anyone is nearby, so they must've dumped this guy here... All Might's coming soon, I should tell him-"

As Izuku moved to make his way to his meeting place with All Might, a piece of unbalanced debris fell from atop of the trash pile he was hiding behind. He instinctively jumped with a yelp, managing to dodge the debris. Unfortunately, this alerted the creature, who started to walk over to investigate.

The creature caught sight of him, and when Izuku started to run, the creature followed. Fortunately, it wasn't too fast to catch up, and Izuku used the labyrinth of garbage to lose it. Unfortunately, it didn't seem like it was running out of stamina anytime soon, and it was also plowing through the garbage piles, leaving holes where it had ran through.

Izuku stole glances back to check how far back that creature was, trying to figure out how to lose him more effectively, when he tripped over a piece of discarded wood, falling over. As he turned over to stare at his imminent demise, a familiar red-and-blue blur rushed in and punched the creature right in its face(?).

"Phew, almost didn't make it there," Build said, shaking his hand. "Good thing I was nearby when I got that Smash alert. Don't worry, kid, I'll take care of this."

Build dusted off his hands, and resumed his assault on the creature. "He said 'Smash alert', so I guess they're called Smashes," Izuku thought, wondering if the name was some sort of mockery of All Might's attack names.

The Smash swung its arms wildly at Build, who dodged and block each attack. Build summoned his drill-sword-gun weapon thing from his belt, and started swinging the spinning weapon against the Smash, though it was largely unaffected by the attack.

"Oooh, you're a tough guy. Guess that'd make you the Tough Smash, then? Hm... nah, how about Strong Smash?" Build quipped, as his weapon disappeared. The Strong Smash growled, and managed to swing one of its fists into Build's gut, knocking him back a bit. "Oof, nice punch you got there! Gotta keep my distance. Guess I'll try using a little more firepower. And I did want to see how this Trial Match worked, anyways."

Build pulled out the bottles in his belt, and pulled out two new ones, a red one shaped like a fiery bird, and a gray one shaped like a gatling gun barrel. He shook the bottles and untwisted the caps before inserting them into his belt and cranking it.

"Phoenix! Gatling! Are you ready?"

"Build up!" Build exclaimed, as his armor changed. The red parts of his armor stayed red, though this time a slightly different shade. His right shoulder pad was now shaped like a flame, with a matching flame-like blade on his right forearm, and his left eye was now shaped like a fiery red phoenix. The blue parts, on the other hand, were now a gunmetal gray, his left shoulder armor resembling the stock of a gun, and his right eye was gray in the shape of a gray gatling gun. On the upper left side of his chest was a red flame, and a series of "bullets" came across his chest like a bandolier. On his back was a segmented cape that looked like flaming feathers.

The Smash charged towards Build, who jumped into the air and flew over it. A gatling gun barrel formed underneath the flame decoration on his chest on the gunmetal gray parts of his armor, and fired a salvo of fireballs at the Smash, who was too slow to dodge any of them, and was knocked back by the explosions.

"Hm, combining Gatling's ability to create gatling barrels on its side with Phoenix's firepower is a good idea, but considering that creating them takes a lot out of me, I should really work on creating a weapon for Gatling instead," Build said as he landed. "Back to RabbitTank to finish this, I suppose," he added, as he changed forms once again, though Izuku could've sworn he heard Build mutter something like "Figures I'm forced to use the only Best Match I have."

The Smash started to charge towards Build, who began to crank his belt once again, setting up the same attack from yesterday. Once Build finished cranking his belt, he ran away, only to create another hole that he jumped in, where a pillar immediately shot out of, while a sloping graph formed that pinned the Smash in place.

"Ready, go! Vortex Finish!"

And just like before, Build slid down the slope, and landed a powerful explosive kick that sent the Smash flying back, weakened from the attack. Once he landed, he pulled a blank bottle out, and twisted it cap, sucking in light particles from the Smash. Where there was a Smash, was now a tall, muscular white-haired boy with six arms and a mask. But the clothes he was wearing, the torn-up white cotton shirt and pants, made Izuku freeze. Those were the same type of clothes he was find in after his kidnapping.

"And that's that," Build said, closing the cap of the now-bulbous brown bottle. He looked at the former Smash, who groggily looked around before falling over unconscious. "Hm, I guess people with mutant-type Quirks don't experience temporary Quirk loss like emitter or transformation types do. At least, surface level wise. Should move this guy outta here, I personally wouldn't want to wake up in a trash heap like this."

"H-hey," Izuku said, as he watched Build pull the youth towards the street, grunting as it seemed like the boy was much heavier than he anticipated. "Build! Can you tell me what's going on with all of this!?"

"Sorry kid, I don't really have the time to explain this, especially since you probably wouldn't comprehend it. Gotta need a big brain like mine to do that," Build said, a bit dismissively. "Plus, can't let any of those pro heroes, police officers, or Guardians see me. Seriously, those were some real hurtful words they were all calling me on the news."

"B-b-but-"

"Just gonna ask that you tell this guy what happened and all, since I'm sure he'll have questions when he wakes up. Hope you have a good day, doing whatever you were planning to do at a messed up beach," Build said, as he prepared to jump away.

"Can you tell me anything about the bat, cobra, and spanner men!?" Izuku blurted out, freezing Build in his place. "I... I feel like whatever has been happening, those guys are at the center of it all. And well, you kinda look like them, since your eyes are shaped like objects, and their visors are, too, and-"

"You've seen the bat man?" Build said, hesitantly. "And there's a cobra man and a spanner man, too?"

"Y-yes!" Izuku said, nodding his head. "Well, it's more like a dream I had of them. You see, I was kidnapped some time ago, and I was found dumped in a completely different place with some amnesia. Ever since then, I've been having the same dream of being trapped in a box, being experimented on, and those men... So do you know who they are!? D-did they turn me into a Smash!? So did you save me, too!?"

Build stood there, thinking deeply.

"No. I didn't. I remember all of the people who I saved, and I'm sorry, but you weren't a Smash. But maybe... maybe you're just like me. They experimented on me, too, and they took all my memories somehow," Build said, walking over to Izuku. He grabbed him by the shoulders, excitedly. "This is perfect! I finally have a real lead! Most of the people I save either don't remember what happened quickly enough to tell me or stay unconscious for too long that I can't hang around! Can you tell me anything else? What did their lab look like? Did you check the area where you were found in? Can you lead me there!?"

"Well, I-" Izuku began, when he heard All Might calling for him.

"My boy, I heard a commotion, and I came as fast as I could-" All Might said, jogging towards them in his skinny form. He paused when he saw the Kamen Rider. "You!"

"Oh, that's not good. I can't stay around here to let him call a pro hero or something," Build said, looking at the skinny man. "Listen, we gotta go, kid."

"Wait, what do you mean-"

Build picked Izuku up in a bridal carry, and jumped into the air, leaving All Might behind. Izuku screamed, as Build hopped from rooftop to rooftop. "W-wait, calm down! There has to be a better way to handle this!"

"Relax, when we're done, I'll take you back to your dad, no sweat," Build said, causing Izuku to sweat at the thought of having All Might as a father (which would be pretty amazing, to be honest).

"H-he's not my dad! I mean, I wish he was," Izuku said, as Build jumped from another rooftop. "I mean, not that I don't love my dad, he's just working overseas a lot."

"Well, whoever that guy is, there's no way he's catching up to me, so we can-"

"HAVE NO FEAR, FOR I AM HERE!"

Build turned to look back, and saw a rather furious All Might following him, practically seeing the steam coming out of his ears and mouth. "Holy smokes, it's All Might! How did he find me so fast!?"

Frantically, Build booked it out of there as fast as he could, knowing that yesterday's escapade from All Might was sheer luck, and that he had a head start and hid in an alleyway far too small for someone as big as All Might to go down into easily. But something seemed off about All Might today. For one thing, he was wearing the same set of clothes that other guy was wearing, and there was no way that guy could've called for a pro hero that quickly. So the only other reason for him to have found him-

"Wait... that makes too much sense," Build muttered, as he kept taking glances back at All Might. "No wonder I didn't see him back then, he was in that other form!"

"Uuuuuh what other form?" Izuku said, trying to play stupid, though he quickly realized that it would've been smarter to just not say anything.

"Well, that just confirms my suspicions," Build said, as he came to a stop, as All Might slammed onto the rooftop.

"You have 10 seconds to let young Midoriya before I-"

"Okay, okay, there's a huge misunderstanding. I didn't mean to kidnap him. So let's just talk, All Might," Build said, as he let down Izuku gently.

"And why should I listen to you?"

"Well, for one thing, the only person that saw me take him was that skinny guy at the beach, who was wearing the same clothes as you," Build said, crossing his arms. All Might froze in place, though his furious glare didn't die down. "I'm taking a wager that that's your real form, and that the one you're in right now, is simply just your Quirk expanding your muscles and power."

"And what about it?" All Might said, clutching his face. "That doesn't change the fact that I can still fight you."

"I know, but let's trade secrets," Build said, pulling out his bottles and taking his belt off. His armor dissipated, leaving behind a man with short black hair, a shirt that's split down in the middle, one half red, the other half blue, jeans, one blue shoe and one red shoe, and an oversized brown overcoat. "My name's Sento Kiryu, and I'm Kamen Rider Build. You don't need to hold that form for this conversation, because from my observations, you're actually running low on energy, and that steam's coming off of you is an indicator that you're reaching your limit."

"How did-"

"From my observations of videos and news articles about you, you've been slowing down on your hero work to about three hours a day, which how I reached that conclusion, and a couple of phone videos from random bystanders have shown a faint steam coming off of you at times. And since that's the case, I assume that this kid-" Sento said, pointing to Izuku, and a tuft of hair started to rise up, "-must be your apprentice or something, since you clearly knew who he was, and he seemed to know about your skinny form. Am I right so far?"

"... How did you think all of that up?" All Might said, apprehensively, though the hesitation did give Sento the answer he needed.

"Well, my Quirk's not called 'Big Brain' for nothing, you know," Sento said, his smile slightly smug as he pointed to his head.

Sento Kiryu! Quirk, Big Brain! It enhances his intelligence to genius levels, grants him acute observational skills, and gives him an impeccable memory! Any book about science or language he reads, he instantly understands and comprehends it all! Unfortunately it doesn't work quite as well with other fields such as the fine arts or history, or anything that he doesn't have an actual interest in really, and any theories he makes he still has to test to confirm!

"But in any case, you can trust me, All Might. I don't have any intentions of telling anyone your secret," Sento said, crossing his arms. "Besides, I already let you in on my own big secret, so it's all balanced, right?"

All Might stared at him, and sighed. At that point, he released his transformation, and returned back to his skinnier form. "Alright, you win. What do you want from young Midoriya, Mr. Kiryu?"

"I want to help him find out about the men who experimented on him," Sento said, as he sat down on the floor. "I've been trying to find those guys for months, since they did the same thing to me, and all those other Smash subjects."

"'Smash'? Is that what they're called?" All Might said, raising an eyebrow.

"Yea. Didn't come up with the name myself, not sure why they're called that, beyond the fact that they have a preference to break as many things as possible," Sento said, scratching his head. "Anyways, back on subject. I only took him to see if I had a lead on where those guys are. You said you don't remember where they took you, right? But you do remember where they left you"

"Y-yea," Izuku said. "I can lead you to where I was dumped. We might be able to find some clues or something there."

"That sounds like as good a plan as any, I suppose, considering our limited knowledge," Sento said, getting up. "I'll take you there, and we'll scour the area as much as possible to find anything."

"Count me in, too," All Might said, pointing to himself. "If anything goes wrong, you could use the backup, even if you're the Kamen Rider. Besides, Midoriya's my responsibility as, er, my apprentice. If I can't help him out with his biggest mystery, than what kind of mentor would I be?"

"Alright, but we'll probably have to take a car there. I only have a motorcycle, and I don't think having All Might be seen with the Kamen Rider would be good for your image. At least not until they stop saying all that stuff about me."

Fortunately for them, All Might still had the truck that he and Izuku were using to take the garbage they were collecting to the proper dumping sites, while Sento used his motorcycle (the two of them still couldn't believe that his smartphone could turn into one with one of those odd bottles he used) to follow them. With Sento being out of earshot, All Might said to Izuku, "While Mr. Kiryu seems to be trustworthy for now, we mustn't let him know about the true nature of One for All. At least as long as possible. Who knows how long it'll take for him to figure out with that Quirk of his?"

"I understand, All Might," Izuku said, nodding in agreement. "Still, it's hard to believe that someone like him exists. The only other person I know about with super intelligence like that is U.A.'s principal, Nezu."

"Well, I wouldn't say that. There was Takumi Katsuragi."

"You mean that former Institute scientist that was murdered a couple months ago by the ex-boxer and sidekick Ryuga Banjo?"

"Yea, that's the one. He was a smart guy, all things considering. Sure, there were a couple of malicious rumors here and there I heard about him, but he seemed to be an alright guy when I met him. Still, if there was anyone who could've figured out about One for All on their own, it probably would've been him."

"So it's of utmost importance that we can't slip up, because who knows what'd he do with that information?" All Might ended, as Izuku vigorously nodded. While Sento did seem to be an alright, if somewhat smug, man, they really didn't know him that well yet to trust him with any more secrets. So for the moment, they'll just have to deal with him knowing about All Might's true form.

After nearly an hour and a half of driving, the three of them made it to the abandoned warehouse that Izuku had been found at. Once they got out off their vehicles, each one of them began to scour the area for any clues they could find. In between examining each empty crate and barrel, All Might and Izuku would ask Sento questions about his powers and weapons, since they were nothing like anything they've ever saw.

"So what are the little colored bottles you use with your tech?" Izuku asked, looking through a utility closet.

"Oh, those are Fullbottles. They're the purified essence of the Smashes that I collect," Sento said offhandedly as he shifted through wooden pallets. "Each of them provide different abilities based on what they are. So far I have Rabbit, Tank, Phoenix, Turtle, Gatling, Lion, Pirate, Beetle, Jet, Vacuum, and Refrigerator. And whatever this one's gonna be. By inserting them into my belt, the Build Driver, I can transform into Build."

"And your forms?" All Might inquired, as he walked up to the second floor catwalk.

"Oh, those are easy. There are the Trial Matches, which are the ones where I match random Fullbottles together. And there are Best Matches, where I match a Fullbottle with the correct corresponding partner to get the strongest abilities out of both. Not to say that Trial Matches aren't useful, though, since there can be certain situations where using a Trial Match is better than using the Best Match. At least, theoretically. I've only found RabbitTank."

"How did you come across this tech? Did you make it yourself?" Izuku said, sighing as it became apparent that there really wasn't much they could find here.

"Oh no, I wish. I mean, I got the hang of it, and I do build a lot of the newer tech, but it's all thanks to-" Sento began to say, when they were interrupted by a sudden growl. They fell silent, searching the area for the source of the growl, while he put his belt on.

Suddenly, a Smash broke through one of the walls, knocking it down. Sento immediately inserted the Rabbit and Tank Fullbottles into the Build Driver, and cranked it as fast as he could. Tubes came out of his belt to form runners with his armor, the red Rabbit half in front, and the blue Tank half behind him.

"Rabbit! Tank! Best Match! Are you ready?" the belt asked him. Sento answered with a firm reply. "Transform!"

"The Moonsault of Steel! RabbitTank! Yeahhh!" the belt announced, as the runners slammed onto Sento, forming his Kamen Rider armor.

The Smash that appeared before them seemed to be especially robotic in nature. Its armor-like flesh exposed wire-like skin, with two powerful clamps for hands, and a wide torso that had large headlight-like lightbulbs on its chest, and exposed gears whirred in its abdomen. Its shoulders had gun-like attachments, and its head was robotic-like, almost like a Guardian's head, but with three large telescoping eyes on it, and wire-like "hair" growing out just above its neck. It growled, and ran towards Build, firing bullets out of its shoulders.

Build summoned his sword-drill-gun weapon, and tried to smack the bullets away, but a few of them moved around his weapon and struck him in the back.

"Ouch! Homing bullets? That's not fair!" Build said, as the Smash got close and started to swing punches at him.

"Don't worry, I'll help!" All Might said, as he began to bulk up.

"No, don't! Only Build's able to safely extract the Smash essence! Fighting without a Build Driver won't work!" Build said, blocking an attack from the Smash. "Appreciate the offer, though!"

All Might frowned, as he released his muscle form. He didn't like that. He didn't want to be helpless when others are in danger, especially knowing that whoever this person was didn't deserve to be turned into a monster like this. The future hero-in-training clutched his fists, too, trembling as he watched Build fight.

Build pulled his weapon apart and reattached the blade to the handle to form its gun mode, and began firing at the Smash. It jumped away to avoid the spray of bullets, though it didn't dodge of them in time. Grunting, the Smash stuck out its right arm, and its hand fired out like a grappling hook, grabbing and pulling Build's gun away.

"Huh, neat trick. Guess I should call you the Claw Smash, since you're like those claw games with hands like that," Build said, as the Claw Smash launched its left arm to grab him. He backed up just enough to be out of its range (though he did stumble a bit on an errant plank of wood that was behind him), and pulled out the Fullbottles out of the Build Driver. "Since this place is so messy, let's clean up with a little bite, shall we?"

He inserted the Lion and Vacuum Fullbottles into the Build Driver. "Lion! Vacuum! Best Match!"

"A Best Match? Finally, something new! Let's go!" Build said excitedly, cranking the handle as a new set of equations appeared around him and the Smash. Runners came out of his belt, with the front forming the marigold Lion half, and the back forming the turquoise Vacuum half. "Are you ready?"

"Build up!" he announced, as the runners slammed onto him, forming the Best Match's armor. "The Mane Cyclone! LionCleaner! Yeah!"

The right arm of LionCleaner was a shining marigold color, with a lion head-shaped gauntlet and a tail-like protrusion on his shoulder armor. The turquoise left arm, on the other hand, in place of a hand, had a vacuum cleaner-like attachment over it, with a tube attached to it that led directly into a case-like shoulder armor. Build's right eye resembled a vacuum cleaner, while his left eye resembled a lion's head. "I've found the winning formula," he said, as he traced the "nozzle" of his eye with the back of his right index finger before doing a hand flick.

The Claw Smash roared, and it launched both its clamp hands towards Build. He stuck out his left hand, and fired a powerful blast of wind at them, sending them flying back into the Claw Smash's face, knocking it backwards.

"Wow, the Long Range Cleaner's blowing power's more than doubled!" he stated giddily, as the Claw Smash prepared to fire its shoulder guns. Before the bullets could curve around him, Build sucked all of them up, and fired them back, knocking the Claw Smash down. "Same with its sucking power. Now, let's try out the Lion half!"

Build clutched his right hand, and a bright yellow energy began to coat it. As the Claw Smash struggled to get back up, Build threw a punch, sending a small energy projection of a lion's head into it, knocking it back down.

"So I can launch punches with the Gold Lio Gauntlet! That makes up for the charge time."

Build used the Long Range Cleaner's sucking function to pick the Claw Smash up, and punched it with the Gold Lio Gauntlet. After repeating this a couple of times, the Claw Smash seemed to be sufficiently weakened. "Alright, time to finish this!" Build announced, cranking the Build Driver's handle. "Ready, go!"

Build stuck out the Long Range Cleaner, and it began to suck the Claw Smash towards him, though much faster than before. Once it got stuck in the nozzle, Build lifted it up, and shot it into the air. A large lion head projection formed around his right arm, and he sent it flying into the Claw Smash, exploding upon contact. "Vortex Finish!"

When the Claw Smash landed, Build pulled out an empty bottle, twisted its cap open, and pointed it at it, sucking up the glowing Smash essence. When he was finished, the Claw Smash turned into a young curvaceous girl, roughly around Izuku's age, with pink dreadlocks, and yellow eyes with crosshair-like pupils. Unlike the boy from earlier, she was dressed in a school uniform, though it was different from Izuku's. All Might entered his muscle form, deciding that if he couldn't help bring this girl to normal, he could at least comfort her.

"Ugh... what happened?" she muttered, looking around. "Where... where am I?"

"It's alright, you're safe now," All Might said, with a hearty laugh. "You were turned into a monster, but thanks to the Kamen Rider, he helped turn you back to normal!"

"The Kamen Rider!?" the girl said, almost excitedly, but she immediately clutched her head and moaned. "Oh man, I'd be more happy if it weren't for this splitting headache."

"Do you remember what happened?" Izuku asked, looking at her. "Did you get taken to some lab or anything?"

"I was going home to mine, yea."

"Yours?"

"Yea, I wanted to work on the gear I wanted to submit for my admission test for U.A's support course, but then this weird gas trail came out of nowhere and hit me in the chest. Next thing I know, I'm here," the girl said, rubbing her head. "Eugh, can we go somewhere else? I need to nap for like, a few days..."

Before anything more could be said, a squadron of robotic Guardians came into the empty warehouse, surrounding them. "Guardians?" Izuku said, puzzled. "I didn't think there was a troop nearby..."

"Something's not right about them," All Might muttered, as he walked to the girl's side, picking her up in a bridal carry. Build clutched his right hand, letting energy charge in it. Even Izuku got into a fighting stance, though admittedly it was a bit awkward looking since he hadn't practiced yet. The apprehensive group stared at all of the Guardians when they heard a strange slow clapping.

"Congrats on beating that Smash so quickly," a garbled voice said, as the Guardians parted to reveal the voice's owner. He was dressed in a gray leather-like undersuit with black armor, with black pipes resting on his back like a jetpack of sorts, and thin black spanner-shaped bars rested on his gauntlets. On his chest was a blue spanner, and his helmet's visor likewise matched it, while a smokestack-like horn rested above it.

There was no doubt about it. This was the spanner man from Izuku's dreams.

"Honestly I thought that one was pretty excellent, having the ability to generate its own ammunition that could home in on its targets and having grappling claws like that, but I should've figured that the Kamen Rider would've figured out how to beat it," the spanner man said, crossing his arms. "Still, that was some valuable data I gathered from that fight."

"You-! You're the spanner man!" Izuku said, tensing up.

"Spanner man-? Hm, I suppose I haven't introduced myself yet. My name is Meta Bolt," the spanner man said. "As for why I'm here, I suppose I wanted to test the Kamen Rider myself for once. As intriguing as the Smash experiments have been, I figure gathering data firsthand would be more, enlightening."

"You..." Build said, his hands tightening. "You've been turning people into Smash, just to experiment?"

"Well, that's not the only reason. However, I don't think I'm in a place to say why yet. My coworkers wouldn't appreciate me spilling everything too quickly," Meta Bolt said, as a strange gun materialized in one hand and a strange blade in the other. The blade was the oddest, having a valve on the side and a scope on top for some reason. He added with a chuckle, "Hey, maybe if you beat me, I'll tell you more."

"People aren't just yours to experiment!" Izuku said, the joints in his fingers popping from his clutched hands. "You're turning people into monsters! What kind of scientist are you, ruining people's lives like that!?"

"Tch, like a kid would understand," Meta Bolt said, popping his neck. He gestured to Izuku with his gun, and spoke to the Guardians. "Hey, take him. We can probably do more experiments on him. Was definitely a waste to just let him go like that."

Izuku tensed up, ready to use whatever little super strength he had gained to protect himself. But All Might was having none of it. "Mr. Rider! Can you handle fighting this villain and rogue Guardians by yourself? I'll take young Midoriya and the little miss here to safety!"

"Be my guest!" Build said, rushing towards Meta Bolt as All Might lifted Izuku onto his back, and leaped out of a broken window. "Let's go one on one, Screwloose!"

The Guardians started firing their guns at All Might, but Build used the Long Range Cleaner to suck up their bullets, and fired them right back at them, damaging them enough to shut down. He returned his attention to Meta Bolt, who had closed the gap between them, and swung down his blade onto Build who blocked it with the Long Range Cleaner's nozzle.

"Well done taking out all those Guardians at once!" Meta Bolt grunted, as sparks flew between their weapons. While Build was distracted with this, Meta Bolt fired his gun right into Build's stomach, knocking him back with a small explosion, following up with more shots. "But I'm on an entirely different level than any Guardian or Smash!"

Build grunted, and charged energy into his right fist, launching several small but quick energy punches. Meta Bolt stopped each one, either by firing into the energy punch before it could get close to him, or by knocking it aside with his blade. On the last one, Build charged longer, and fired a much stronger punch towards Meta Bolt. He simply thrust his blade into the punch, dispersing the energy in a large explosion, which kicked up a great amount of dust, obscuring both of their visions. Meta Bolt didn't have time to blow the dust away when he saw the cloud being sucked in by Build's Long Range Cleaner, and with it, his blade, which landed point first into the nozzle.

"That takes care of that," Build muttered, only to notice Meta Bolt charging towards him, firing bullet after bullet at him. Build blocked each one, only for Meta Bolt to grab his blade's grip, though it was still stuck inside the Long Range Cleaner.

"Smart of you to disarm me like that," he said, complimenting Build. "Too bad there's more to the Steam Blade and Transteam Gun than that!"

To Build's surprise, the grip popped off the blade, only for Meta Bolt to insert his gun in between the blade and handle. The combined weapon announced "Rifle Mode!", much like Build's own Build Driver, and Meta Bolt spun the blade's valve. "Ice Steam!"

Meta Bolt fired his weapon into the Long Range Cleaner, as frost formed inside and out of it, clogging it. He kicked Build, pulling his weapon free from him, and fired more freezing bullets at Build, knocking him to the ground.

"Not bad, Build," Meta Bolt said, as he tapped the weapon's barrel against his shoulder. "You're a lot better than I thought you were. However, this is my victory."

Meta Bolt pulled out a Fullbottle of his own. It had the unmistakable silver image of a spanner on top of a light purple bottle, with a silver cap. Meta Bolt shook his Fullbottle, and inserted it into his weapon, and spun the valve once more. "Devil Steam! Spanner! Steam Shot! Spanner!"

"Like hell it is!" Build said, cranking the Build Driver's handle furiously. "Ready, go! Vortex Finish!"

Meta Bolt fired a large energy beam in the shape of a spanner, which splintered into smaller ones that started to home in on Build. Build pointed the Long Range Cleaner at Meta Bolt, firing the ice that was jammed into it at the spanner bullets. The resulting explosions kicked up a giant cloud of smoke and dust, and when the air cleared, Build had disappeared, having used the explosion to mask his escape.

"Hm, smart move, using the ice I stuffed his weapon with to counter my attack," Meta Bolt said, sneeringly. "Still, he's got a long ways to go before he's really useful..."

Sento scowled as he ran. He couldn't believe it. He blew his chance to find out who he is, and what was going on with the Smash experiments. He didn't even find out the name of the organization behind all this. He only knew about one member. One member. And he said coworkers, clearly referencing the bat man and cobra man. And if they're anything like Meta Bolt, they're bound to be extremely skilled and powerful.

Once he made sure that he was far enough away that Meta Bolt couldn't have followed him, he leaned against the wall, and slammed his fist against it in frustration. He checked his phone to see if All Might or Izuku sent him a message, before realizing he never traded numbers with either of them. "Man, I'm really off my game today..."

"There you are, Mr. Kiryu!"

Sento turned around, and saw All Might (in his skinny form) and Izuku jogging towards him. "Thank goodness we found you! All Might left the girl at a police station, and we headed back as soon as we could, but you and Meta Bolt were gone! What happened?"

"... I lost," Sento admitted, shaking his head. "Meta Bolt was a lot stronger than I anticipated, and all I could do was escape. He's still out there, no doubt planning to keep turning people into Smashes."

He slammed his fist against the wall again, scowling. "Stupid! I had our answers in front of me, and I couldn't do anything to get them!"

"Hey, it's alright, Mr. Kiryu," All Might said, giving him a reassuring pat on the shoulder. "There will always be more chances to figure out things. Why, you got me and young Midoriya helping you out! So we're bound to be able to get answers again soon."

Sento took a deep breath, and sighed. They were right. He's only been at this for a while, so there's still a lot of time to solve this mystery. "You wanted to know how I got my hands on this tech, right? Well, I figure it'd be best if I took you to the man himself. That, and I'm famished. I'll treat you to dinner at the place I live at, since he's the one who's been housing me, too."

"That sounds fantastic, but can we go get the truck? We left it back there, and it's a rental..."

"Bolt, what were you thinking?" Rogue said, as he rapped his fingers on the armrest of his chair. "I already made it clear, don't recklessly use the Transteam Gun's rifle mode for testing Hazard Levels. Not only that, you chose to engage with the Kamen Rider, as well as All Might. Why?"

"Like I said before, I wanted to test to see how far Build's progress has been going along, and what better way than to fight him?" Bolt said, flippantly. "While it's definitely enough to fight a mindless Smash and the average Guardian, he's no match for a Transteam Gun user. As for All Might, well, he seemed more concerned about taking those kids out of the way, so I didn't bother going after him outside of having the Guardians shot at them for a bit."

"Hm, a fair assessment," Rogue said, as he stroked the chin of his helmet. "Still, you're becoming far too careless in your approach after finishing the modifications for the Transteam Gun's rifle mode. I'm putting you on the bench until you learn more restraint."

"Sir Night Rogue, Sir Meta Bolt, if I may interject?"

Bolt and Rogue turned to face one of their Nebula Chamber scientists, who had finished cleaning up it up. "I feel like... calling it the "Transteam Gun's rifle mode" is a bit of a mouthful. Why not shorten it to something simple, like the Steam Rifle?"

"... Who are you to suggest anything?" Rogue said sharply, almost glowering at the scientist who began to back away in fear. "Especially as something as trivial as a name?"

"I think it's an excellent name," a new voice interjected, one that was far more casual in tone than either Bolt's or Rogue's. They turned to face their third member, who, like them, was dressed in a leather-like undersuit, though his was red. His gunmetal armor pipes formed a muffler-like collar around his head, with a green cobra insignia on his chest, and his green visor matching it. "Besides, the 'Transteam Gun's rifle mode' is a mouthful."

"Stalk..." Rogue said, turning to his fellow leader. "Of what do I owe the pleasure of seeing you?"

"Just checking up on things. Gotta say, I'm impressed, Bolt. You really outdid yourself on improving the Steam Blade's Nebula Gas injection capability by giving it range. To think, we can now just fire a gas bullet and bam! Instant Smash."

"Need I remind you that the Smash are not our final objective?" Rogue said, crossed.

"Yea, yea. Speaking of which, we should slow down on the whole experimentation thing for a while, since we're starting to catch the attention of the media. Orders from our good sponsors," Stalk said, patting Rogue on the shoulder, who pushed his hand away. "Or at least be more discreet."

"Hmph, fine. That does line up with my order to Bolt to stay put, not to mention give the scientists time to recalculate and modify the Nebula Chamber so we can use it on mutant-types more easily. The Nebula Chamber's door is getting cracked a bit," Rogue said, as he inspected the top of the Nebula Chamber. "Can't be too careful... Since you finished your modifications with the, um, Steam Rifle, I suppose I'll put you in charge of it for now."

"Fine," Bolt said, crossing his arms. "Anything for Faust, I suppose."

"Well, if that's settled, I should get going," Stalk said, dusting off his hands. "Keep up the good work, you two. After all, there's a big future ahead for us all."

Stalk laughed as he walked off, leaving his two compatriots to stew in their thoughts.

ORIGINAL AO3 NOTES:
- So while for the tags I'm using the versions of the names where they have a "u" after "o/u", I'm gonna just omit that entirely in the fanfic itself. So instead of Ryuuga Banjou, it's Ryuga Banjo. And yes, I'm also gonna use "first name last name" instead of "last name first name", too.
- Sorry, but I'm gonna be using the English versions of the Fullbottle names, as well as "Transform" instead of "Henshin", if only to keep the fanfic as easy to read as possible.
- While the Kamen Riders are transformed, I'm going to only refer to them as their Kamen Rider name and not their regular name to establish it as an identity they take up. Now this won't be the case when they transform for the first time or when the narrative hasn't revealed it yet, like how Sento was only ever referred to as "the Kamen Rider" before he revealed his name as Build.
- A thing I'm changing is gonna be the stickers/labels on the caps. For the most part, they're just gonna be the Build symbol with no letters to hint at what the Best Match is. While that's good for a toyline, it kiiiinda makes it silly how Sento can't figure out what the Best Matches are when the answers are sorta staring in his face.
- Relating to the Fullbottles, they're all going to be the same in color scheme. I'm gonna make the "Hokuto" and "Seito" Fullbottles solid colors instead of them being the "colored image only", since in this fanfic, they all need to be collected instead of only the Touto ones. Nothing else is changed about them, though. They're still the same shapes as they are officially.
- Mild Spoilers for those unfamiliar with Kamen Rider Build: Something I realized I should've clarified last time, I'm gonna be replacing two Best Matches. One is MerryChristmas with F1Saurus, for obvious reasons, and PenguinSkater with CrocodiCon, since for whatever reason that was never an "official" Best Match despite the existence of the Crocodile Crack Fullbottle and the Gear Remocon, so I wanted to make a better fit, and as cool and fun of an idea a skateboarding penguin is, I feel like is one of the Best Matches that's easy to replace.