Author's note: Hey everyone, Fire Slash here and welcome to another chapter of Iron Deku. Once again, I'd like to thank everyone who has shown support for this story. It means a lot to me that so many of you like this story and wish to see it grow. I apologize for the late update, but writer's block had hit me hard. Anyways, if you haven't already, follow my IG (Fire_Slash_Fanfiction). It's the best place to reach me regarding updates on stories.

Well, I've said all that I've had to. Let's get right into the story, review and enjoy :-)

0000

"Hey, nice suit!" Mirio said, waving at Izuku as he and Mei entered the gymnasium. Prior to their fight, Mirio made sure to use his status as U.A's top student to secure the school's gymnasium for their spar.

"Thanks, glad to see you're wearing the costume Mei and I made you," Izuku replied.

"Of course! If I didn't, you'd have to fight me naked and who'd want to do that?"

Izuku shuddered, glad that he chose to challenge Mirio after making his costume. "Not me." He then noticed two people sitting on the benches in the background, a boy with black hair and a girl with blue hair. "Who're they?"

"Ah! Those are my friends and the other members of the big three. Amajiki and Hado, come say hi to Midoriya!"

"Hi…" the boy with black hair shyly greeted.

The blue haired girl was much more enthusiastic about meeting the two new people. "Oh, you're the two who made Togata's costume! Hi, I'm Nejire Hado! How'd you do it?! Do you think you can whip something up for me to help with my quirk?! It's called 'Wave Motion' and it lets me convert my own vitality into energy and release it through shockwaves. It's a bit slow though and that sometimes gets annoying when fighting…"

"Have you tried using shorter bursts?" Izuku asked, his inner hero-nerd coming out. "People using shockwaves forget that even short bursts are powerful. Try pretending you have a gatling gun next time you use your quirk. Maybe aim using your fingertips instead of your pals when using the small bursts if you find it hard to control..."

"Oh no, there's two of them…" Mei groaned, watching the two quirk enthusiasts chat.

"Now you know how everyone else feels when we talk about our hobbies."

"Yeah, but our babies are cute and I know what's going on! How can I not gush all over them?!"

"Wait, you're parents?" Nejire asked, causing Izuku to shake his head.

"No, Mei just calls her inventions her 'babies.'"

"Oh! That's cute!"

"You don't mind if they watch our fight, do you?" Mirio asked the young genius.

"Not at all." In fact, having the other members of the Big Three watching could be beneficial. They could give him tips on how to improve his fighting style. "Hope you don't mind if Mei watches. I need someone to record data for me."

Mirio shook his head. "I don't mind. Well, I'm ready when you are!"

Izuku nodded as Mei made her way to the other members of the Big Three, pulling out a tablet. Pressing the side of his helmet, he caused it to close, protecting his face. He then began analysing his opponent. 'Mirio says that his quirk allows him to phase through matter, but can he also faze light and sound waves? Also, does his quirk alter his body like some other physical based quirks?'

"J.A.R.V.I.S, set all attacks to nonlethal."

"Done."

Upon receiving confirmation from his A.I that his suit will not be killing anyone today, Izuku turned to the blonde. "I'm ready. Oh, and you're free to damage the suit as much as you want. I can always repair it. "

"Alright, here I go!" Mirio began lunging forwards, cocking back a fist back. Izuku however was ready for this, eying his opponent with an analytic eye. He had his suit fire missiles at where Mirio was, hoping to deal some damage to him before making another move. When the missiles hit its target, smoke began forming in the spot where Mirio was, or where he once was. With no grunts of pain, Izuku knew the boy managed to phase himself into the ground before they could even make contact.

'He dodged that?!' Izuku was impressed. His missiles were very quick and there wasn't much room between the two, so for someone to react that quickly to them without a speed quirk was nothing short of impressive. 'Now, where did he go?'

Izuku surveyed the room, looking for his opponent.

*PANG!*

Thankfully he didn't need to search far, for Mirio ended up trying to blindside him with a left hook, but ended up finding out the hard way that punching metal was generally a bad idea.

"Ow! Man, that looked so much cooler in my head…" Avoiding an elbow strike thrown by the young genius, Mirio phased through the ground again.

This time, Izuku decided to take a different approach when dealing with Mirio. Taking to the skies, the boy began to patiently wait for the blonde to pop out of the ground. Once he did, Izuku immediately blasted him in the back with a repulsor ray, causing him to stumble backwards.

"Heh, nice shot," Mirio complemented before ducking into the ground again, avoiding another repulsor ray. Upon popping out of the ground, he made his upper body intangible, allowing for the third repulsor ray aimed at him to harmlessly pass through.

'Seems like his quirk protects him from concussive force… as to be expected. But can it protect him from sound?' Activating his shockwave generators, them taking the form of small megaphones that popped out of his wrists, the boy unleashed a burst of soundwaves at Mirio, who had just reentered and re-emerged from the ground. Again, his attacks harmlessly passed through him. 'Crap. So his quirk makes him phase through everything, including sound, air, and light. He can't stay intangible for long since doing so cuts his access to oxygen, but he's trained so that he can effectively use his quirk in bursts.' Looking down at Mirio, Izuku smiled. 'No wonder he's the top contender for the number one hero slot once All Might eventually retires.' His smile then changed to one of confidence. 'But if I stay in the skies, I can eventually tire him out and win. With a close-quartered quirk like his, I don't see how he can get up here.'

Activating his missiles and the repulsor rays on his hands, Izuku unleashed an onslaught of attacks. To his credit, Mirio managed to nearly avoid everything Izuku threw at him, popping in and out of the ground, but sadly, even he couldn't come out unscaved, a repulsor ray nailing him in the arm and chest.

'He knows about my limit,' Mirio thought to himself as he watched the airborne Izuku launch more attacks at him, causing him to have to avoid them yet again. 'I can't hurt him in my current state. Looks like I'm going to have to go all out!'

Phasing back into the ground, Mirio launched himself upwards, flexing his muscles as a faint yellow aura began covering him. Gaining immense air time, Mirio threw a punch at a stunned Izuku, slamming him back to the ground. Standing up, Izuku shot a repulsor ray at Mirio, who blocked the shot by forming an X with his arms. A small grunt informed Izuku that his attack did indeed hurt the boy, but it didn't do much.

'The hell? Intangibility quirks shouldn't do that. This seems more like a molecular alteration quirk. He's able to strengthen his body or make it intangible by messing with his molecular structure. People probably mistook him for having an intangibility quirk, which is possible if he learned how to increase his strength at a later point in his life.'

"You're one of the only people who made me have to go all out, so congrats on that," Mirio said to Izuku. "But you won't be able to beat me at FULL POWER!"

Mirio rushed forwards, dodging a repulsor ray and moving at speeds greater than before. Izuku tried to throw a punch, but Mirio caught it, throwing a quick jab to Izuku's stomach in response.

'Ack, I felt that! J.A.R.V.I.S, tell me how much he can take!'

"He can take attacks from your suit utilizing its full power, including punches and repulsor rays," J.A.R.V.I.S informed Izuku as the young boy avoided a punch thrown by Mirio. Sadly, Mirio managed to nail him with a well placed hit to his face.

'Alright, activate all systems.'

"All systems are a go."

Hearing this, Izuku put his hand up, catching a punch thrown by Mirio. He then swung, sending the boy flying back a few feet.

"Good one." Entering the ground to avoid another repulsor ray, Mirio resumed close combat with Izuku. As they fought, Izuku learned some new information about his opponent. Mirio and his suit were equal in strength with Mirio having a slight edge over him. Second and most important, close-quarters combat was his specialty. Now, Izuku was no slouch himself, having had training in martial arts, but Mirio was in a whole different world. Nearly every punch and kick thrown by Izuku was countered or avoided by the blonde and most of his punches managed to hit Izuku despite his attempts at blocking. It was because of this that Mirio ended up knocking Izuku onto his back with a leg sweep.

"Do you surrender?" Mirio asked, holding him by the neck. Immediately, Izuku began racking his brain for a way to get out of this situation. Unfortunately, he was trapped. Mirio could simply phase through any attempts at hitting him with the suit's weapons, especially with his speed being enhanced by his quirk. As such, it was game over for him.

"Yeah. I surrender."

Helping Izuku up on his feet, Mirio let out a hearty laugh. "HAHAHA! That was fun! I haven't sweat like that since my last spar with Amajiki. We should do this again sometime."

Opening his helmet, Izuku couldn't help but smile. Sure, he lost, but all that meant was that he needed to improve in some areas. The fact that he was receiving a compliment for his fighting prowess from Mirio of all people was an achievement of its own. "Sure, if you don't mind. I could always use a partner to help me improve." Izuku turned to Mei. "Did you get everything?"

"Yep!" The pink haired girl chirped, moving towards her friend now that the battle was over. "I got the whole match recorded, I'll send it to you once I get home."

"Thanks. Hopefully I can incorporate some of that data into my Mark III suit."

"Well, we did your thing, now can we go back to the lab so we can work on baby number 336?"

"I did promise you that, didn't I?" Facing the Big Three, Izuku shot them small smiles. "Mei and I have some business to attend to. It was nice meeting you guys and thanks again for the match Togeta!"

"No problem! See you guys later!"

"Bye!

"Bye…"

Unknown to Izuku, his spar with Mirio would end up kick starting his and Mei's career in becoming U.A's 'Dynamic Duo' two years down the road…

0000

"So, how'd you make that suit?" Mirio asked Izuku the next day as the Big Three and future Dynamic Duo ate lunch together. Izuku and Mei usually ate lunch alone, not really knowing their classmates well enough to try to make friends with them, so when Mirio showed up with his friends and asked if he could sit with them, it came as a shock to Izuku.

"Classified," Izuku responded, biting into a sandwich.

"I've been trying to get his blueprints for a while now," Mei added.

"Aw, I really wanted to know how you were able to shoot those laser thingies…" Nejire said with a pout.

"Repulsor rays," Izuku corrected her. "They're an invention of mine. The repulsor rays come from arc reactors, which I also use to power the suit. There are two small ones in the palms of the Mark II and a large one in it's chest."

"What powers them?" Tamaki (Izuku ended up learning that Amajiki's full name was Tamaki Amajiki from Mirio when they first arrived at their table) asked in a soft voice.

"Again, classified. I did invent it myself though."

"So let me get this straight, you managed to build your own energy source by yourself?" Mirio asked.

"Yeah."

"That's crazy!"

"Heh, thanks. Speaking of crazy, there's something I wanted to discuss with you about your quirk."

"Sure, what's up?" Both Tamaki and Nejire couldn't help that Mirio looked a bit nervous.

"You said your quirk was an intangibility quirk, but you displayed superhuman strength and speed. Are you sure your quirk's now a molecular alteration quirk? It explains why you're able to do things others with similar quirks aren't."

"You might be right," Mirio stated, secretly releasing a sigh of relief. "It's only recent that I discovered this part of my quirk and I can't use it all the way or else I break my limbs. I can only use up to twenty percent without hurting myself."

'His quirk is still developing?' Izuku found that hard to believe. Mirio was already as strong as Death Arms, one of the powerful heroes in terms of raw strength. If his quirk was still developing, then he could someday end up being as strong as All Might. "Well, they say a quirk is a muscle, so keep on working on mastering it and someday you'll get the hang of it. Even though you're already a ball of walking muscle, I'd suggest maybe adding some more muscle? It might give you more to work with… I'm not sure. I like to think that I know my stuff when it comes to quirks-"

"He's an encyclopedia when it comes to them," Mei interrupted, causing Izuku to playfully roll his eyes at his friend.

"-but molecular based quirks are always strange."

"Oh, you like quirks too?" Nejire asked excitedly. "I love quirks! I think they're so cool and there's so many out there! We should talk about quirks sometime! You probably know some cool ones, like Amajiki's. His lets him turn parts of his body into whatever he has eaten and he can eat nearly anything! Well, it still has to be in his digestive system, but as long as he doesn't use the bathroom before fighting, he's fine."

Hearing her talk, Izuku had J.A.R.V.I.S record the conversation through his Mark Mini. In the meantime, Mei was giving the shy boy a hungry smile.

"You can turn into anything you eat?"

"Yes?" Tamaki didn't like the sinister look on Mei's face.

Izumu immediately caught onto what his friend was alluding to. "Mei, no. We're not turning Amajiki into a human mining center."

Mei's eyes gleamed with madness. "But imagine how many materials I can get to create new babies!"

Izuku groaned as he began to explain the concept of human decency to Mei as the Big Three watched, Nejire and Mirio laughing in amusement while Tamaki ushered a silent prayer, hoping for some protection against the young inventor.

0000

Izuku took out some blueprints and frowned. He was almost done with the Mark III, but today had him thinking about a new design. While the Mark III was to be twice as strong and durable as the Mark II, it still wouldn't be enough to fight against people like Mirio and All Might.

'Better to start a new design, especially after what happened today.' According to his mother, one of the hero classes was attacked by a group of villains, his mother being told about it from Katsuki's mother. Thankfully, no one was mortally wounded. Damage was severe however as a villain managed to leave a nasty scar on one of the students and another completely decimated their teacher. Said villain would go up against All Might and almost win.

There will be times where Izuku will need to go against villains like the monster All Might fought and he needed to be prepared. He needed a suit that could contend with them physically.

Sending a few hours in his personal lab, Izuku managed to create a draft of the blueprints for this new type of armor. It would take a decent amount of time to create, but it should be fully completed by the end of the year.

'What I need is something that could be used to bust down those threats.'

Putting down his blueprints, Izuku exited his lab, ready to get some sleep.

0000

Author's note 2: A shorter chapter than usual, I know, but it sets the stage for some major events.

Regarding the Mirio fight, yes, he has One for All. Now, I'm not sure if Mirio could best Izuku without it. His quirk's crazy strong, but he can't do anything to Izuku. Mirio's quirk is like Katsuki's, in normal circumstances, it lets them wreak havoc. However, both have a weakness that anything sturdy enough shuts them down completely. He isn't like Kitty Pryde from the X-Men who can phase people through objects. Meanwhile, Izuku's reflexes can't keep up with Mirio's speed and skill, so hitting him would be hard. In the end, it would be a battle of endurance. Who would win, I'm not sure :-/. However, I will say that Izuku will be getting a rematch later in this story, after upgrading his suit of course.