Reincarnated in the World of High School DxD

Kazuki Fuwa collapsed to the ground after losing both of his arms.

"Gurrr...!"

The pain was overwhelming, but it was more like a searing heat. The proliferating cells quickly sealed the wounds, stopping the bleeding. The pain gradually subsided, but it would take time for the injuries to fully regenerate.

As he rose to one knee, he looked up to see the Demon King, Sirzechs Lucifer, standing calmly above him.

The figure of the Demon King was both terrifying and beautiful, a true embodiment of overwhelming power and dominance.

"This is as far as you go. I would like to say you fought well, but you didn't quite meet my expectations," Sirzechs remarked coolly.

Kazuki gritted his teeth in frustration.

"Unfortunately, you've failed. I cannot entrust Rias to you. As her big brother, I command you to leave her side."

"..."

"You may have defeated Kokabiel, but it seems he has grown weak indeed. To be defeated by someone like you..."

"…SHUT THE FUCK UP~."

"Hmm? Did you say something?"

"Shut up! Close that uselessly smooth-talking mouth of yours right now!"

"...Oh? You still have the strength to talk back," Sirzechs said, his lips curling into an amused smirk.

"Of course I do. I'm not dead yet. If you want to gloat about defeating me, wait until I'm actually dead!!"

With a roar, Kazuki unleashed a barrage of Head Beams. The beams were, of course, blocked by Sirzechs' defensive field, but they served as a distraction.

The blinding light from the beams momentarily caused Sirzechs to lose sight of Kazuki.

Seizing the opportunity, Kazuki exploded with magical energy, propelling himself into the air above Sirzechs.

"Take this!!"

With a gravity-infused Gravity Kick, Kazuki's right leg shot down like a meteor, aimed directly at Sirzechs.

"Guhh!"

"Raaaaah!!"

The Guyver's kick finally shattered Sirzechs' defensive field, and the two plummeted to the ground.

"Now!!"

Guyver seized the opportunity to go on the attack, and was opening with brutal precision. A spinning double kick—Sen Tsujii—whirled like a sawblade through the air.

He feigned a high roundhouse before abruptly shifting trajectory, his boot slamming into Sirzechs' solar plexus with the vicious —Shiden— technique.

Without pause, he launched into a front flip, driving twin axe-heel strikes—Halberd's Edge—into the Maou's collarbone.

The relentless parade of Fuwa Enmei-ryu kicking techniques overwhelmed even the Demon King, forcing him to his knees for the first time.

"...Inconceivable."

Grayfia, ever composed, stared wide-eyed at the scene. Had she ever witnessed Sirzechs in such a state? Certainly not since becoming his wife.

This duel had been meant to test Kazuki's resolve and gauge the truth behind his mysterious Guyver powers. The boy was undeniably strong, his martial prowess combined with the alien armor Guyver's enhancements already rivaled a High-Class Devil's might, as proven by his solo annihilation of Kokabiel.

Yet the Guyver's origins remained shrouded in danger. Kazuki himself claimed ignorance of its source, and such unchecked power was a threat to all. Thus, Sirzechs had intended to push him to his limits: if Kazuki surrendered under pressure, so be it. If he resisted, they'd have their answer.

However, when the lid was opened, Guyver's power exceeded the Demon King's expectations.

Who would have imagined that Kazuki would be able to corner Sirzechs?

"Kazuki... he's incredible," Rias murmured, her earlier frustration forgotten as she watched her lover battle with feral intensity even against her powerful brother.

'—But he's still holding back.'

Grayfia clenched her fists, clinging to the belief that Sirzechs would ultimately prevail. She prayed he wouldn't be forced to unleash his full, catastrophic power.

"Huh, heh heh heh..."

Sirzechs suddenly laughed, blood trickling from his split lip as he rose from the crater. "Well played... I retract my earlier doubts. Consider my approval granted—your engagement to Rias stands."

"The fuck I care about your approval," Kazuki snarled.

Sirzechs tilted his head, crimson eyes narrowing. "Oh? Does Rias mean so little to you, then?"

"Wrong," Kazuki growled, the Guyver's alien armor humming with predatory resonance. "Titles mean shit. Even if you deny the engagement, I'm not letting Rias go. She's mine. I'll butcher anyone who tries to take her. That woman belongs to me now."

Sirzechs stared, uncharacteristically speechless, before erupting into thunderous, bloodthirsty laughter.

"Fuh… kukuku… AHAHAHAHA—!!"

"Kazuki…❤"

Rias's eyes shimmered with heart-shaped desire, her cheeks flushed. Grayfia sighed, pinching the bridge of her nose. 'Of course, the heiress would melt over being claimed like a trophy, but this was borderline obscene.'

'Did I spoil her too much…?'

The silver-haired maid exhaled again, questioning every life choice that led to this moment.

Sirzechs' laughter finally subsided, his crimson gaze sharpening. 

"To boldly declare my sister as 'your woman' to her brother… You've got balls, Kazuki. I'll give you that."

"You mocking me?" The Guyver's mega-smasher crystals flared, bio-plasma crackling.

"Praise, actually," Sirzechs purred, hellfire swirling around his fingertips. "Very well—Rias is yours. But as men… we settle this properly."

"Yeah, I agree." Kazuki's armor hissed, tendrils of organic cabling writhing hungrily.

The Demon King spread his obsidian wings, ascending into the crimson-stained sky. "I'll unleash my full-power [Magic-Extermination Cannon]. Survive it… and the win's yours."

"Demon Lord Sirzechs Lucifer, himself. I will show you the even greater power of Guyver. Come! "Kazuki barked.

Sirzechs' hands glowed as hellish energy condensed into a searing orb, compressing until its magical density dwarfed his prior attacks. The air itself screamed under its weight.

Kazuki's Control Metal flared. A colossal, cocoon-like structure erupted behind him—a grotesque chrysalis of pulsating bio-armor.

"Guyver: Giant Body Armor!!"

The cocoon shattered, crimson plates engulfing his body. The Guyver Gigantic towered, its chest armor splitting open to reveal a swirling singularity of annihilation.

"What the—?!" Sirzechs' composure cracked.

"Call it my 'Balance Breaker'," Kazuki's voice boomed, distorted and primal. 

"Now eat this, Lucifer—!"

Sirzechs's Demon-Extinction roared toward Kazuki, a tsunami of obliteration. The Gigantic's chest cannon answered, unleashing a star-forged beam:

"GIGA… SMASHERRRR—!!!"

The two great forces collided. For a heartbeat, reality held its breath—before the Giga Smasher devoured the hellfire, erupting into a supernova that vaporized the barrier and lit the heavens ablaze.

Sirzechs stared at the scorched sky, a grin splitting his bloodied face. "…Magnificent."

~~~~~~~~~~~~~~

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