Location - Unknown (RWBY)

Keith groaned as he opened his eyes. Aches of pain echoed throughout his body.

"Where am I? What happened?" He murmured before he remembered that he was in the Red Lion. Then he remembered what had happened before. The Plan to take down the Galra. The Battle at Zarkon's mothership. The Lions being scattered throughout the universe because of the Wormhole malfunction caused by the Dark Druids.

"You all right, Red?" Questioned Keith. A weak growl told him that while she was fine, the Red Lion needed some time to reboot. Precious time that could bring the Galra right to his doorstep.

'Well, sitting here's not going to do anything. I really need to find out what the world that I have crashed on looks like. And hopefully, find help in getting a beacon to show my location to the Castle of Lions.' Thought Keith as he decided to do just that.

A few minutes later...

"Here goes nothing." Muttered Keith as the lion's jaw began to open so that he could see the world that he landed in. Given his luck, it could have been a barren wasteland or a poison bog. Heck, it could have even been a Galran outpost.

It was nothing like he expected.

"What the….?" Said Keith as he stepped out into a moonlit glade surrounded by pine trees and bushes. Grass grew under his feet. He surveyed the area, seeing a few trees crushed by the fall of the Red Lion. The HUD in his helmet said that the air was breathable like Earth's, so he removed his helmet and took a deep breath of air, savoring the fresh air that flowed into his lungs in the moonlit glade. It was like he was back on Earth. Then he looked up to the sky and his eyes widened.

'Almost.' Sweatdropped Keith as he gazed at the shattered moon of this world instead of the reassuring full moon of Earth.

"Okay, well I'm not on Earth, sooo… is this Neverland or the land of Oz or something like that? Because I've heard of those places and none of their moons looked like some Weblum had gouged it out." He joked, freaked out by now, while gazing at the shattered moon of…..whatever world he had landed on.

A rustle in the bushes alerted Keith to his surroundings. Changing his bayard into weapon mode, he walked forward slowly.

"Ummm…. Who's there?" he called out, trying to not startle whoever or whatever was in those bushes. Heart racing, Keith pushed the bushes aside with his free hand to reveal….

A rabbit.

Or to be more correct, a black-furred rabbit's backside facing him while it was hiding in the bush, spooked by the noise created by the crashing Red lion.

And doing a terrible job at it, I might say.

"Phew!" Keith sighed, relieved. "For a moment, I thought that it would be some alien creature that would try to take my head off or a Galran patrol. Come on. Go home, little guy." he stated trying to shoo the rabbit so it would go away.

The rabbit pulled its head out and looked at him. Keith was not scared. Why was he supposed to be?

After all, it was a rabbit.

A god-to-honest, black-furred, bone mask-faced…

Wait.

Since when do rabbits have bone masks on their faces?

And since when do rabbits have blood spatters over their face and blood dripping from their mouths?!

And since when do rabbits have giant, black, bone-armored creatures standing behind them?!

Which is what Keith saw as a giant shadow rose from the bushes behind the not-rabbit rabbit.

It resembled a gigantic black dog, except it had bone armor on its limbs and body, a bone mask covering its upper head with three spikes protruding out of the back, red glowing eyes shining in the sockets of said face, said armor having thunderbolt-like spikes and patterns and crimson sparks coming out of its closed mouth.

The Beast (whatever it was) let out a massive roar before focusing its red eyes on Keith.

"Quiznak." Groaned Keith as the not-dog dog opened its mouth to let out a blast of crimson energy.

A couple of miles away…

A portal opened before someone stepped out of the Portal. This person had messy brown hair and was wearing a magenta shirt, black jeans, black converses, and a maroon trench coat. Around his neck hung a camera and around his waist was a belt with a magenta buckle of some sort.

"Well, well." Mused the man "I never expected to find the kid here. It makes my job way easier than I thought. This is gonna be a piece of cake: just give him the driver and tell him the instructions."

Suddenly, a rage-filled howl rang through the air before a bolt of crimson energy flew up and exploded, illuminating the surrounding area.

"Well, it seems like the kid has got himself into some trouble already." Sighed the man before he took out a card from his pocket. "So much for this being a piece of cake. Well, I'll better help him out before it's too late. Henshin!"

At that, he slotted the card into his buckle before twisting said buckle.

=KAMEN RIDE: DECADE!=

Meanwhile….

Keith was exhausted.

So far, he had crash-landed onto a strange new world, met a killer rabbit with an energy-guardian-dog thing, and nearly got roasted by said thing. On top of that, he was chased by giant not-crow crows, nearly been stung by not-wasp wasps, nearly been bitten by not-wolf wolves, nearly been gored by not-boar boars, and to top it all, not-bear bears had tried to rip him a new one literally.

Yeah, today was a day of firsts for Keith….yet again.

Now, he had given all of those not-earth creatures earth creatures the slip, giving him a few moments to breathe.

Which was what he was doing: slumping against a log and taking in a few deep breaths.

"Phew! What on earth is going on in this quiznacking world?!" He sputtered, looking around nervously when he heard a twig snap. "Must be a deer or raccoon or a…" his voice died out as he saw the same not-rabbit rabbit from before. "Demon Rabbit." He said before looking up and seeing the not-dog dog above his head, red eyes glaring directly at him while the other creatures surrounded him, preventing his escape.

"This is gonna suck, isn't it?" He gulped as the dog opened its mouth to chomp his head off. "Yep, thought so."

=ATTACK RIDE: DECADE BLAST!=

Suddenly, blasts from a gun of some sort began hitting the not-dog dog, causing it to howl in pain and stumble back. It then whipped its head to where the blasts came from, snarling. Keith did the same, turning his head to the clump of bushes on the far right. The bushes over there rustled before someone stepped out of them.

"What the hell….?" sputtered Keith before his voice died as the person walked into the light, thus showing his full body in profile. Said person was wearing a black and white bodysuit that had magenta and black armor on the chest, arms, shoulders, and legs. Said suit had an asymmetrical 'X' on one side of the chest armor. But what caught Keith's attention was the helmet. It had 7 bars running vertically parallel to each other on the face and green eyepieces over those bars. In his right hand, he held something that resembled a book of sorts except it had a gun muzzle sticking out of the front and a handle in the back of it. It could only be…

"Kamen Rider Decade." he murmured, having seen both the magenta Kamen Rider and the cardholder/sword/gun weapon, the Ride Booker, in the Tokusatsu series Kamen Rider Decade. 'What on earth is going on here?!' He thought. 'Did I die and go to heaven or something? Because meeting Tsukasa Kadoya in the flesh is any Tokusatsu fan's dream, goddammit!' He fanboyed silently while the creatures all stared at the new arrival, unsure of what to make of him.

One of the alien wolves charged toward the newly arrived Decade….who simply sighed before blasting the creature apart with a few well-aimed shots of the Ride Booker. He then straightened the handle of the Ride Booker before a blade popped out of the other end, turning the Ride Booker to its sword form.

Decade then proceeded to go on the attack, destroying all of the not-earth earth creatures that had tormented Keith with no mercy. In fact, it was pretty much one-sided. The not-crow crows went spiraling down to the ground, wings sliced off, squawking in pain before dissolving into mist. Meanwhile, the not-wasp wasps fell down to earth, sliced up either in halves, thirds, or tiny pieces before dissolving as well. The not-boar boars were shanked, turning into not-boar boar pork filets, the not-bear bears roared before charging at Decade….before they were shredded apart by the Ride Booker's wild strikes and faded away. The not-wolf wolves, on the other hand, took a good look at Decade….before sending one with a scar on its face away….before charging towards Decade anyway with the giant Thunder-Dog thing leading the head.

None of those not-wolf wolves survived, not even the giant Thunder-Dog creature.

Later…

Keith couldn't believe it.

Not only did he see Kamen Rider Decade in action, but he also got the chance to see him in his Civilian persona AKA Tsukasa Kadoya.

Who was currently leaning on a tree trunk and looking at him while shaking his head after he dehenshined out of the Decade armor.

"Well, I gotta give it to you, kid." he began. "You did try to hold yourself out against those creatures, but you nearly got yourself killed. I mean, what is going on in that pinheaded mind of yours, huh?"

"Well… umm…." Began Keith, not sure what to say. "What were those things?!" He finally decided to say, tired of all the surprises that he had encountered in one day.

"Right, well the creatures that we encountered are called the Creatures of Grimm by the people of Remnant. They're not living in the sense that we are and seek only to kill for killing's sake." Began Tsukasa. "Remnant's the name of this unfortunate world that you crashed upon." he said to a blank-eyed Keith. "Moving on, the crows that you encountered are called Nevermores, the boars are Borbatusks, the wasps are Lancers, the bears are Ursas and the wolves are Beowolves. Anything else that I missed?" he questioned an astonished Keith.

"Well, what about that rabbit and the demon-dog thingy?" He asked, pretty certain by now that Tsukasa could tell him. "And who came up with those weird names for those creatures?"

"Well, I have no idea." Tsukasa replied to Keith's query. "And FYI, I am most definitely not a Grimm Specialist." He replied to a dumbstruck Keith. "Now, I might know the second one and apparently it's the Grimm equivalent of the mythical Japanese creature called a Raiju. Basically, it's a Thunder-Dog-Guardian thing of the god Raijin, the Deity of Thunder in here, but there's no Japanese equivalent in Remnant. Also, it dropped a Fullbottle. That is not normal." He held up a black Fullbottle that had the Raiju Grimm's image on it. Keith took it in his hand and dropped it into his pocket.

"But I'm still drawing a blank on the rabbit thing that you said you saw." Tsukasa sighed. "Anywho, I was sent here by Sento Kiryu to give you something."

"Wait, did you say Sento Kiryu? The Sento Kiryu from the Tokusatsu show Kamen Rider Build?" Sputtered Keith, still not willing to believe anything that was happening right now.

"Yep, I said Sento Kiryu himself, and I meant the egoistical, narcissistic hero of justice as said by Yours Truly himself." Came the reply. "He told me to give you this. And be careful; the stuff that's in here is delicate and Sento threatened to turn me into a monkey if it becomes broken."

At which, Tsukasa hefted a heavy silver briefcase that was not there just a few moments ago and unlocked the hasps before passing the briefcase to Keith. Keith hesitated before taking the briefcase from Tsukasa and opening it.

Inside the briefcase was a purple-colored Build Driver and a set of Full Bottles of varying colors. He gingerly took out the Driver, feeling cold metal instead of plastic before looking at Tsukasa.

"Anyways… how do I use them?" Keith questioned Tsukasa, who sputtered and nearly face faulted at the question.

"Maji De?!" He gawked before going into a rant, tired of Keith's constant questions "Just place the belt around your waist before slotting the Biotic bottles in the left slot and the Abiotic bottles in the right slot, dammit! And use the Drakken and the Chain Full Bottles. They're the only Best Match in the entire set that I know of! And before you ask; I DON'T KNOW WHERE THE OTHER FULL BOTTLES ARE OR WHAT OTHER BEST MATCHES ARE THERE!"

"H-Hai, Senpai!" Keith panicked, scared out of his wits by Tsukasa.

Keith then took a few deep breaths before raising the Driver.

He slammed it to his waist, and from it a purple belt loops around his waist before it locks back into the Driver.

He then raises a red bottle, and a yellow one, and smirks.

"Sa, jikken o hajimeyou ka?" He begins shaking the bottles, and behind him, enormous equations appear in the air.

Tsukasa sighs before smiling to himself

"Well…this never gets old…"

Keith adjusts his grip to move the tops of both bottles, aligning them with their fronts, before slamming the red in the left slot, then the yellow into the right slot on his Driver.

DRAKKEN! CHAIN! BEST MATCH!

As holograms of each indicated item appear, his smirk grows wider and wider. After the announcement of 'Best Match', a slow beat fills the air. His right hand rises and grips the crank on the side, before rotating it. With that, the sound of gears, pistons, and colliders echoes in the air, almost as if they were some sort of music. From the belt, tubes manifest, filling with a liquid that forms two halves of a complete costume, in front and behind him. Keith raises his left hand out as if reaching for something.

ARE YOU READY?

The question echoes in the air before he shouts a single word.

A word that would forever change the fate of Keith and the people who would meet him.

"Henshin!"

The halves slam together, and steam sprays from the whole assembly, as Tsukasa looks upon the new Kamen Rider's complete form.

TOJIKOMERARETA-KEMONO! DRAKKEN-CHAIN! YAY!

The new rider then looked over himself, his eyes widening at the suit and also at the sword that he held in his hands. It resembled the Beat Crosser that Kamen Rider Cross-Z wielded, except the color scheme was red and yellow, and the blade was longer. This was the Drakkensaber.

"Eh? Eh? EHHHHH?!" He shouted, amazed and freaked out. "NANI KORE?! I became a Kamen Rider?! DA FUCK?! THIS IS SO FUCKIN' COOL!"

"Well, thanks for thinking so, and could you please lay off the profanity? It makes you sound pretty uncouth." Remarked Tsukasa. " And you're no egoistical, narcissistic hero of justice; the last thing that I need is another dumb monkey like Cross-Z, or another idiot Fanboy like Grease, or another beardo with no fashion sense like Rogue." He shuddered at the memory of Rogue and his... questionable sense of fashion.

"Well, this is cool! Look at me! I'm Build now, too!" He said.

Tsukasa huffed. "No, you're not."

"Huh? But… But I just used the Driver! I'm Kamen Rider-"

"Prime." Tsukasa's interruption took him by surprise.

"Eh?" That was all that he could say.

"Keith Kogane, you're not Build. You're the Raging Hero….. Kamen Rider Prime."

"Kamen Rider Prime. Hmm…." Wondered Keith. 'Not that bad at all.'

Meanwhile….

The scarred Beowolf knelt in front of a throne, looking at the figure sitting on it who was obscured by shadows.

"Hmm… So that armored person slaughtered all my Grimm?" Said a female voice from the shadows. "And you said there was another person with him? A kid?"

The Beowolf nodded.

"Hmmm, now this could be…. Interesting."

Hello!

Sorry for the delay; got busy with other jobs.

So, most RWBY crossover fanfics start with the chapter Red Like Roses. But that would not explain how Keith got the Primedriver (This is the Build Driver in a Purple Redeco) or how he crash-landed in Remnant in the first place (The Events take place after Season 1 in Voltron: Legendary Defender).

So I created this prequel story and kept it separate from RWBY Canon because it acts kinda like a filler for the series and helps us transition from how he came to Remnant to how he joined Beacon.

Tsukasa Kadoya will have a part in the events to come, and expect more of the Kamen Rider cast to join him as well (maybe…..?)!

Also, the Senpai from Build might make a appearance in this series.

Also, expect more Grimm themed after mythological creatures to appear throughout the series. They are the Smash from Kamen Rider Build in this series (Or an equivalent.)

Up next, we finally delve into the World of RWBY and introduce our main characters!

Kamen Rider is owned by Tokusatsu

RWBY is owned by RoosterTeeth

Voltron: Legendary Defender is owned by Netflix

The rider forms are owned by TajadorCombo, Coeghepher, and DarkTidalWave on Deviantart.

The only thing I own are some OCs, Shippings, and Plotlines.

Thank you and comment down below!