Fire and Paper (BakuDeku Quirk Transfer AU)

Why?

Katsuki waited until Izuku had left before packing up and heading towards the roof himself. For once, he hated spicy food; it seemed to churn in his stomach, making him feel nauseous with every step that he took towards his destination. The bag on his arm seemed heavier than usual, as if he were carrying a bowling ball rather than a tiny, miracle-granting device. He paused at the bottom of the stairs, his legs feeling as if they were sinking into the floor, making every step up the stairs hell. Fucking hell. It's just the damn nerd. I can do this. He told himself, forcing himself up the rest of the way before pausing.

A few feet away, his back turned, stood Deku, shifting his weight uncomfortably and checking his phone every few seconds. Katsuki gulped. Maybe this wasn't a great idea after all. But he'd gone through so much of a hassle already; he wasn't about to back down now. Taking a breath, he opened his bag, taking out the device.

I read it needs DNA in it. What's easy to use? He ran his hand through his hair as he scratched his head in thought, glaring at the device in question. His eyes widened. Guess that fucking works. Yanking out a hair, he forced it into the device, prepping it to check Izuku's compatibility the second it was handed to him. He took another breath. Then a step forward.

"Oi, Deku," he called, hiding the device behind his back. Why, he wasn't sure. Maybe just instinct since it was a surprise?

The green-haired boy turned around, eyes wide. "Kacchan! What are you doing here? I was supposed to meet someone but they never showed up! Maybe it was all a prank. Or someone was trying to tell me something but chickened out. No, probably the first one. No one here really pays me much attention otherwise so it must—"

"You're muttering again, you damn nerd."

"Ah! Sorry!"

Katsuki scoffed. "I've got something for you, dumbass. Close your eyes and hold out your damn hand."

"What? Wait, why do—"

"Just fucking do it, or I'm going home, damn it!" Katsuki had to resist the urge to bring his hands in front of him, to create explosions from them in an angry threat.

"R—right." The boy did as he was told, shaking slightly.

Katsuki sighed before stepping forward and placing the device in Izuku's hand, pressing the button in the process and causing it to let off a small "ding."

"W-what was that?"

"Nothing important. Just fucking trust me for a sec, ok?"

Katsuki watched as the device scanned Izuku's hand. Why does it need my DNA, yet can just scan Deku like this? Why doesn't it need both of ours? He wondered, narrowing his eyes in annoyance at the completely illogical set up. His thoughts were interrupted when the light flashed green. His eyes widened as light flowed from the device and onto the boy, coating him in an orange glow intense enough to be painful, yet Katsuki couldn't tear his eyes away at the sight. Izuku radiated in the warm glow that seemed to pour from him; whether it lasted a year or a second, Katsuki didn't know, and for once, impatient as he usually was, he didn't care.

The light eventually faded. Was it done? Katsuki hesitantly reached out to take back the device. "You can open your eyes now, Deku," he spoke unusually softly, still amazed by what he had seen. "Do you... Fuck, how do I fucking say this shit? Do you feel any different or shit?"

The boy opened his eyes and stared at him before his eyes fell to the device in the blond's hands. "Is that—?"

"Yeah. Your mom tried one of these a long time ago, didn't she?"

Izuku's eyes went wide. "How did you—"

"Who do you think went and got this piece of shit? Now, do you feel any different or was all that flashing light shit just a fucking fluke?" He rubbed his eyes, irritated from staring, and turned back to the boy, only to find him crying. "What the shit?!"

"There— was no light last time," the boy said between sobs. "It— it must have worked. But I don't understand. Whose Quirk is—"

"Mine."

Katsuki wasn't prepared for the hug that ensued, almost getting knocked over by the crying boy as he gripped him tightly, drenching the front of his uniform shirt.

"Thank you. But, why?" The vulnerable voice was but a whisper, full of both wonder and confusion.

Why. That was a good question. Why did I do this? Pity? Katsuki thought to all the times he had bullied Izuku. No. I've never given him pity. "Would you like him if he had a Quirk?" The question echoed through his mind. "I don't know. But if you're gonna be a hero, then you need a good Quirk and everyone knows my Quirk is damn amazing. So you better fucking use it after the hell I went through to give it to you." He activated his Quirk in his free hand, glowering at the boy.

"But I— I don't know how! I've studied your Quirk for years, but that doesn't mean I understand how to use it. I— I never thought there was a reason for me to."

"I'll—" Katsuki paused, contemplating what he was about to say before finally saying it. "I'll fucking teach you the goddamn basics. Obviously you'll never be as good as me but you better pay enough attention to be decent or I'll kick your ass for wasting my goddamn time. And after that, if you still need fucking help like the dumbass you are, we'll... figure shit out."

With that, Katsuki turned and left, a blush on his cheeks, and a single question resonating in his mind. Why?