Author's note – Kishimoto Masashi owns Naruto while Guyver is written by Yoshiki Takaya.
I make no claim to either, I am merely playing in their sandboxes.

Please note that in this AU, a certain event happened three years later than in canon. You'll know it when you see it.

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Naruto The Guyver

Prologue: Archanfel

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Archanfel lowered his weapon and stared emotionlessly at the ashes that had been the test subject prior to it donning the armour. His cape fluttering in the breeze caused by the superheated air rising to make way for cooler air, he turned back to face his masters, the beings who used the armour that they had tested on one of those he had been created from, beings who had lost an entire command ship and no less than four defence ships to a single, untrained primitive from the native species that they had adjusted to create an army that they could command.

This must not be allowed to happen again.

Archanfel didn't respond as the mental communication had not been aimed at him. Instead, he de-powered his weapon before shifting his grip on it, raising it above his head so that it could be reclaimed. As it was lifted out of his hands, Archanfel felt… something, sense of loss… of potential.

New weapons will be forged.
Ones that cannot use the suits.
What happened today was… not as anticipated.

"Guyver." Archanfel whispered, giving the thought its closest approximation in the limited form known as speech.

Prepare the Grand Hammer for deployment and evacuate the planet.
Leave whatever cannot be taken within the four-day, we must depart.

Archanfel blinked as the Advent floated away towards the distant ships, then he turned and looked at the distant shape of the damaged ship that was sinking into the lava, its self-repair systems already working to seal the hull. Nearby lay the unit that had granted the deceased primitive powers beyond the Advent and Archanfel began moving towards it, then he froze as a bolt of lightning from the clouds forming above lanced down and struck it. For a moment, it glowed like an earthbound sun, then it seemed to vanish as its dimensional storage systems overloaded and Archanfel felt another emotion.

Anger/Irritation.

The ground trembled as the first ship of the Advent lifted off and Archanfel watched them leave. Motionless, he waited for the last of his former masters to depart, uncaring that the process took over five days, and only then did he begin to move.

The ship that had been destroyed had been the one holding all of the records about where the caches of technology had been placed ready for use in building outposts. He had only learned the locations of five of them and several had probably been retrieved, but as long as at least one remained intact, he could use it as the base of his plans.

If any of the other units had been left in one of the caches, he would find it and would claim the power for himself. He would create weaker versions and use them to build his own army, his chosen ones leading the primitives to the stars, unleashing them against the Advent and seizing all that had once made them great.

But he could not do it alone.

He would need assistants. Helpers. Beings that he could empower so that they would be able to oversee events for him.

A slight movement in the skies above him caught his attention and, gathering his willpower, Archanfel rose into the skies, ascending into orbit with practiced ease even as his eyes searched for what had…

There!

A massive meteor, easily large and fast enough to ruin the planet… his planet, should it hit!

The odds of such an event occurring briefly crossed his mind, then he realised that it had been dispatched by the Advent, that the beings on the world below him would never become a threat.

That his plans, embryonic though they still were, would never come to be.

This could not be allowed.

Archanfel closed his eyes in concentration, although his senses could still feel the approaching planet-killer from its very mass. Calling upon all his reserves and channelling all the power of his twelve crystals, he unleashed it in a blast of ravening gravitonic fury that lanced out and slammed into the meteorite.

For almost a full day, Archanfel maintained the blast, slowly ripping the meteorite apart before he finally felt its central neutronium core dissipate. Drained and exhausted, Archanfel let himself descend into the atmosphere once more, realising that although he had save the origin of his eventual triumph, the cost had been high.

One of his crystals was cracked and he could feel it sapping his energy.

For the first time since he had been created, Archanfel knew what it was like to be tired.

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Prologue: Naruto

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Naruto grumped as he crawled through the bushes at the base of the Hokage monument, searching for a spot out of the cold drizzling rain.

He growled as he thought about the bully who had destroyed his room and the matron who had blamed him and thrown him out.

How was a six-year-old meant to…

Naruto's thoughts cut off as he found himself looking at a small cave one barely waist high to him, but it was shelter and without hesitation, he wriggled into it.

The inside of the cave seemed very smooth. A ninja would have recognised it immediately as artificial, the walls and roof formed by Doton techniques.

A paranoid ninja would have noticed the security seals.

A prepared ninja would have anchored a rope to one of the trees outside and had team-mates ready to pull him out at an instant's notice.

Naruto wasn't a ninja, despite his desire to be one.

As such, it was a total shock to him when the ground beneath him suddenly opened, dropping him down a long chute and into a reinforced glass pen inside a large chamber lit by seals set into the ceiling. Unsteadily rising to his feet, Naruto leaned against one of the walls for support, only for the glass to slip out of the decayed fastenings and shatter on the ground.

For a long moment, Naruto held his breath as he waited for the inevitable angry shouting, but to his surprise, nothing happened.

Carefully avoiding the worst of the glass, Naruto moved around the room, staring at the odd shelves and their contents. Strange liquids, internal organs and even less-identifiable objects floated in various jars, then he blinked as he saw something on a low shelf, something that intrigued his curiosity.

The object looked like a disc almost as wide as his head and maybe as thick as his hand could stretch. The outer part of it was comprised of three metal sections, coloured an odd yellow/blue/green while a metal sphere was nestled in the middle. Holding the sphere in place and filling the inside of the armour plates was what appeared to his young eyes to be a long coil of thick, braided ninja wire.

With the utter fearlessness of innocence, Naruto reached out and grabbed the disc, pulling it towards him. As it came off the shelf, Naruto belatedly realised that it was heavier than he had thought even as its weight pulled him down, striking it with his forehead.

"Owwwww. What the…?"

The sphere glowed with an odd white light that attracted his gaze, then his eyes widened as he realised that the wires were moving. Releasing the disc, Naruto started to scramble away from it, but he was too late as the wires erupted from the disc and wrapped around him, then plunged into his flesh.

"AAAAAARRRRGH!"

Naruto's cry of agony echoed in the dusty room as he collapsed, writhing in agony that threatened to overwhelm him, then after what seemed like an eternity of suffering, the pain vanished.

For a long moment, Naruto simply stared at the ceiling, then he frowned. The ceiling seemed… sharper than before and if he concentrated on the light-giving seals, he could see the pattern within them, the way that they took ambient energy and guided it through channels, changing it in the process to produce illumination.

As he stared, he could almost feel the concepts behind the seals slotting into place in his mind, including ways to make them more efficient or brighter. Shaking off his odd reverie, Naruto sat up, then found himself staring at a pair of armoured feet.

Feet that were attached to legs.

Legs that were attached to him!

Naruto scrambled backwards, hitting the wall, then he froze again, this time staring at his hands which were encased in the same sort of odd armour, armour that…was… the exact colour of the disc that had attacked him!

Naruto slowly flexed his fingers, then his legs. Pulling himself to his feet, he looked round, then paused with a new realisation.

He was taller!

Somehow, the armour (whatever it was) had made him grow up!

"Awesome!" He whispered with a wide grin. Spotting a door, he slowly walked over to it, getting used to his longer legs, and grabbed the handle.

CRASH!

Naruto stared at the fragments of the door in his gauntlet, then at the wreckage of the wooden door around his feet.

"Oops." He muttered to himself, then he carefully stepped over the broken wood and looked up and down the corridor.

"Aha!" He realised. "Air is flowing from over there… how did I know that?"

After a long, introspective moment failed to provide him with an answer, Naruto mentally shrugged and moved along the corridor, climbing the curving flight of stairs at the end while frowning at the odd tickling sensations and the plink and tank noises that he kept hearing. Glancing behind him, he froze at the sight of dozens of senbon and kunai littering the ground, weapons that had not been there earlier.

A brief impact made him look down just in time to see another kunai bounce off his chest. Moving with a speed and surety that he didn't know he had, Naruto grabbed the falling weapon, only for the blade to break in his grip. Dropping the fragments, Naruto resumed his interrupted journey until he found himself facing a door. As he reached for the handle, he paused again, staring at his hand.

"Oh man, my armour!" He realised. "Those temes will want it… huh?"

The armour seemed to fragment and leap off his body, vanishing behind him. Naruto spun round, but there was no sign of it.

"Hey!" He shouted. "I want my armour back!"

A surge of energy lifted him into the air and he felt the armour plating slide back into position. Landing gently, he looked at his armoured fists and grinned.

"Way cool! Armour off!"

Once again the armour vanished and Naruto smirked.

"Niiiiiiiice!"

Reaching out, Naruto pushed against the door in front of him and when it opened, he slipped out of the base and back into Konohagakure.

But he would return, so that he could find out what else his newfound armour could do.

After all, it was magic armour.

Just like in the stories that he had overheard while hiding outside the window of the main room after the matron locked the door to his room and he had to climb out of the window.

He had magic armour.

That made him… a hero!

But he had to think of a cool name for when he…

That was it!

"I… am Hiro!" Naruto whispered to himself with a smile as he turned to look at the door as it swung closed. Pushing against it, he smirked as the concealed door re-opened and then he let it close again.

It was a secret place and now it was his secret place.

"Uzumaki-san!"

Naruto spun round as a masked individual with silver hair landed just in front of him.

"There you are." The ANBU said in a relieved voice. "The Hokage was worried about you."

Naruto blinked. "Who?"

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Naruto smiled as he looked round. He would never have to go back to the Orphanage, he now had his own apartment, one paid for by the village to make up for how he had been treated at the orphanage!

His own apartment and his own secret hideout for storing all his cool ninja stuff in!

Life was finally looking up.

With a grin, Naruto started looking round to see if there was somewhere that he could use as a secret exit… aha!

The air vent in the bedroom wall was large enough that for a while at least, he would be able to crawl through it, letting him come and go as he pleased.

During the day, he would be Naruto and he would be Hiro by night.

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Author's afternote:- Yes, the three last words were a shout-out to a certain webcomic called Hero by Night.
It's worth a look.

As for other Naruto/Guyver crossovers, there are only 3 at the time of writing.

Deus Ex Machina - id 7674807 - 9 chapters (60K words/abandoned 2012) (Edit - Story deleted, reason unknown)
Naruto the bioboosted ninja - id 7852442 – 3 chapters (6K words/abandoned 2013) {Meh…}
Guyver: The Ninja Battles - id 5430555 - 1 chapter (under 2K words/non-starter 2009) {No. Just no}

Yeah, this prologue alone is longer than that last one…

Edit - 18/07/15 - New story posted by Lionheart61 on 16/07 - Naruto The Bio-Booster Armour - (in progress) {minor editing issues}