"So it's like a minimum activation energy? Like with catalysts," Katsuki clarified.

"Exactly," Deku responded. "Your quirk requires a much higher level of energy for activation, whereas mine doesn't really need any. That's why you broke my arm. You tried to use as little energy as possible, but your minimum to activate your quirk was plenty enough to hurt you with mine."

"So.. what do I do?"

"That's the thing. It's going to be a lot harder for you to go way below what you ever use than it was for me to go over what I normally do. It's like… when you play the drums. It's a lot easier to teach someone quiet to hit harder than it is to teach someone loud to play quietly. So I think you're going to need to practice controlling the energy you put into the quirk before you try to incorporate any movements. If that makes sense."

"It does." A nice thing about Deku was that he'd always been smart. Not as smart as Katsuki was, obviously, but smart enough that exchanging notes or ideas was always effortless. They never had to battle to be on the same wavelength whenever Katsuki bothered to listen. "So I'll just fucking lower that feeling of power in my hand until I feel like it's, what, one fifth of what I normally do?"

"Probably more like one tenth."

"Fuck, okay. So one tenth from my lowest. And I can compare the feeling to when I broke your arm too and try to get a sense from that. Should be fucking easy."

Deku smiled warmly. "You'll kill it."

Katsuki pushed down his desire to growl an insult at the kid sitting across from him in the athletics field. The nicer Deku was to him the more it made him want to retaliate and he was trying to curb that instinct. 'He's just being stupidly supportive because he would be for anyone. He's not singling you out. He's not trying to shit on you.'

"So I guess I'll just be doing my own thing at first," Katsuki said.

"Yeah," Deku responded, "although I'd be down for no-quirk sparring. It'd probably help to be in tune with my body if you're going to try using my quirk."

'Oh, I'm fucking in tune with your body,' Katsuki thought, trying not to think about the first thing he did once he got his cast off and had his right arm, which was thankfully still usable, available to him again. Being a dude sucked sometimes. "Let's fucking do it." Katsuki stood and began stretching his shoulders out, paying special attention to his recently healed arm. Big or fast movements still elicited pain, but he'd been told to keep moving as much as possible as long as he didn't go overboard again. He stood in front of Deku and put his fists in front of him.

That was how Katsuki's quirk training started. Aizawa had given in way too easily when Katsuki came to him to ask if he could use Deku's quirk as long as he had the nerd teaching him. He knew there was something up with how little trouble he got in and Aizawa always seemed to have some fucking scheme to get him and Deku to get along. Well he got his fucking wish.

For the first few days, Katsuki and Deku would meet after class and spar until they either both got tired out or they got too competitive and started doing real damage—it took a lot of convincing the next day to make the class drop the idea that they had beat the shit out of each other for real. Katsuki would spend the rest of his time trying to focus on translating the heat he normally felt in his palm into a level of energy in his mind, and then trying to decrease that level. It was a weird thing to conceive for Katsuki, like trying to quantify the volume of his voice when it was so quiet he couldn't hear it, but after hours and hours of trying to feel something and maybe feeling something and, oh wait, that was definitely a feeling, that had to be a feeling, he started to get the hang of it. Eventually, he had to pretend to use his own quirk to remember what his original minimum was because he had managed to go so much lower.

So after he felt sure—really sure this time—that he could activate the quirk without shattering Deku's bones, they set up the same dummy that Aizawa had originally wanted him to knock down. Deku sat near him again, looking to him with those same expectant eyes, but this time Katsuki kind of wanted to meet those expectations instead of defy them.

Feeling foolish about squaring up with his hand cocked for a flick, Katsuki scowled at the dummy. He closed his eyes, put energy into his index until he could just barely feel a kindling warmth, and snapped the finger forward. Pain immediately reverberated through his entire hand but he was too genuinely awestruck at the heavy figure in front of him shooting off across the field to care very much. Katsuki whipped his head to look over to Deku, needing some sort of confirmation that he actually just did that with a flick of his finger, and grit his teeth to prevent a smile forming when he saw Deku looking up to him with a proud grin, like he was thrilled that Katsuki finally realized he could do what Deku knew he could all along.

Deku held up his hand and Katsuki high fived him with adrenaline fueled strength. "Holy fuck, Deku," Katsuki said, failing at containing his smile.

"I know," Deku said with a laugh. "It's stupid, isn't it? I really would still be nothing without it."

Katsuki flinched at hearing that reasoning from Deku's own mouth. "Deku, I didn't fucking mean…" he had though. He had meant it. "I was being a dick."

Deku laughed again, airy but resigned. "Maybe. But you were right. It's not like I would've gotten into U.A.'s hero department without a quirk. Look," - he put his hands in front of him in defense - "I'm not trying to start anything. I'm just saying I was lucky to get the quirk I did and I'm glad this situation has serendipitously let me share it with you. So, anyway!" Deku clapped his hands together. "I've been thinking about the next thing we should have you do and I think I have a really good idea. But we're gonna need some help."

Katsuki found out who Deku wanted help from when he came to the field the next day and saw himself talking emphatically to Kirishima. Deku was throwing easy kicks and punches while explaining something and Kirishima caught them all, nodding with an excited look on his face. Deku stopped his barrage and spoke a few words to which Kirishima nodded. The red-haired tank had to take a few steps back to balance himself after Deku sprung into him, hugging him happily. Kirishima grabbed his shoulders and pushed him away slightly, acting unusually bashful for the shitty-haired idiot and looking back and forth from Deku to Katsuki as he noticed him approach.

Katsuki clenched his fists. "What's this idiot doing here?"

"Wha- 'this idiot'?" Kirishima repeated. "Dude, we lived together for a month." Once Katsuki had begun to learn Deku's quirk properly, Aizawa had Kirishima move back into his own room. "We shared a bed. I told you I loved you, man. Really, you shouldn't even be calling me Kirishima anymore, we're in first name territory, Katsuki." Kirishima said everything with a hint of sarcasm, but not enough to keep Deku's face free of surprised embarrassment, which in turn made Katsuki feel heat creeping up his neck.

"He didn't tell me he loved me," Katsuki said quickly, trying to clarify. "Okay, actually, fuck, he did, but he didn't mean it like that. And don't fucking call me that, dude."

"How about Kacchan?" Kirishima pressed.

Anger rumbled in Katsuki's throat. "Don't."

"Okay, okay, holy shit, dude. Don't do that with Deku's face, it's like a baby seal is mad at me, I don't feel comfy with this. I just came to help you out."

"Help out how?" Katsuki growled.

Deku stepped in between the two other boys, protecting Kirishima from Katsuki's sudden anger. "He's gonna be your sparring partner from now on. He can take my quirk even if you accidentally go too hard. It'll be good training for both of you. I want you to attack and Kirishima to defend, neither of you using a quirk. Then, randomly while you guys are going, I'll name an attack for you to do, like 'right hook' or 'left roundhouse', and you have to do that move using my quirk and Kirishima has to activate his to defend. It should really help you to get used to flexibility and control with my quirk."

Katsuki hummed to himself. "So like… fucked up Simon Says."

Deku smiled at Katsuki's understanding. "Yeah. I think it'll be fun."

Katsuki looked at Kirishima, feeling a weirdly strong desire to hit him hard with Deku's quirk. He smiled, agreeing with Deku's sentiment.

"Ready up, Kirishima," he spoke with a sneer, cracking his knuckles.

The first week of training with Kirishima caused both attacker and defender pain and embarrassment. There were multiple times every day when Deku would call out a type of punch or kick and Katsuki would commit hard but fail to activate the quirk, crumpling his knuckles or foot into Kirishima's hardened muscles. Kirishima would have a smug smile on his face until Deku called out a command again and Katsuki succeeded but Kirishima failed to strengthen himself in time and wound up being blasted ten feet back, groaning on the ground. For every time they both succeeded in tandem there were roughly four failures. At first, every time either of them got hurt, Deku would shout his concerns, asking if they were okay or if they wanted to stop, but by the fifth day Katsuki and Kirishima were yelling at him for laughing when one of them crumpled in pain for the tenth time that day. Their sessions went from ten minutes at the start to verging on two hours by the time the third week rolled around, both students making fewer mistakes every day. Weirdly enough, the longer the sparring sessions got the longer they all stayed out on the field afterward. First they'd just trade critique's on each other's performance—Kirishima yelling at Katsuki for going easy on him, Katsuki yelling at Kirishima for hitting him back after a particularly nasty blow, and Deku trying to keep the peace by complementing both of them—but eventually they'd find themselves talking about class or other students or whatever was going on in each other's lives.

"So what's going on with you and Uraraka, Deku?" Kirishima asked one day as they all ate onigiri on the field, almost making Katsuki spit out his mouthful.

Deku looked to the ground. "What d'you mean?" he asked innocently.

"I don't know. You guys are always together. She goes to your room sometimes. Just feels like maybe there's something more than friendship going on." Kirishima smiled slyly to Deku whose face was getting redder by the second.

Katsuki rubbed his eyes, desperately wanting to exit the conversation. The last thing he wanted to hear about was Deku's fucking love life.

"Kirishima," Deku whined, "please, it's not like that. I just… like talking to her."

"So when you two are alone in your room you're just talking?" Kirishima prodded in disbelief.

"Yes!" Deku yelled. "Some things are… private. I don't know. I just ended up confiding in her about some things and now we meet up to talk about it sometimes. That's all. Really." Deku was slowly curling into a ball, hugging his knees and burying his red face into them.

"What can you tell her that you can't tell us?" Kirishima asked like they'd all been best friends from birth.

Katsuki was actually interested in the answer to that. He had known Deku basically from birth. He really thought he knew everything about the kid. What the hell was worrying him that he confided in Uraraka for. All Might or his mom Katsuki would understand, but Round Face?

Katsuki got his answer when shy eyes barely glanced in his direction before Deku responded.

"It's nothing, really. I'd rather not talk about it is all."

'Right,' Katsuki thought. Deku had started being close to Uraraka the night Katsuki blew up on him. Their late night meetings were probably Deku crying to her about how much of an asshole he was—how much of an asshole he'd always been. Deku normally put up a front like he wasn't bothered by Katsuki's treatment of him, especially around their classmates, but that night any traces of that front had disappeared, leaving anger and resentment in its wake. It would be impossible for Deku not to carry deep animosity towards Katsuki somewhere underneath all of that forgiveness and care. Kid had to let it out somehow and Uraraka had taken up that mantle.

Guilt murmured in Katsuki's chest. They probably had a great fucking time talking about it, too. Round Face had made it clear that she didn't like him. He could easily picture them sitting on Deku's bed, exchanging awful anecdotes about him, although he could only imagine it with Deku in his rightful body. Uraraka would tell one of her stupid fucking jokes about him and they'd laugh and she'd put a hand on his leg and he'd freeze up like the dumb fucking idiot he is, never having realized that she liked him and was probably thrilled to have an excuse to go to his room late at night. Maybe she'd have been holding back for so long that she'd skip the pleasantries, sliding her hand up Deku's thick, muscular thigh before lifting his shirt, a pale, firm abdomen peeking through as eager hands unbuckled his belt. "K-Kacchan," Deku would breathe, panicked but excited.

"Well, um, anyway," Deku began, snapping Katsuki out of his thoughts and slipping on his backpack, standing, "I'm just gonna head back, it's getting kind of late."

"Hold up, Deku." Katsuki stood quickly. "Spar with me."

"Huh? Why?"

"I'm tired of thinking so much while I fight. Plus Kirishima isn't supposed to hit back. C'mon, no quirks, I just wanna brawl for a bit."

Kirishima stood and raised his hands. "Fine, guess I'll leave then. Don't kill each other, you two." Kirishima headed back to the dorms with a wave.

"You sure, Kacchan?" Deku asked hesitantly. "You've been doing a lot lately. You should get some rest."

"Deku, seriously, shut the fuck up and come at me." Katsuki was weirdly restless after talking for so long. He wanted to slam Deku to the ground and keep him there.

"Okay, okay, jeez," Deku spoke like he was annoyed but smiled as he walked up to Katsuki. They tapped fists and settled into their fighting stances.

Katsuki moved first, sending a fist straight to Deku's face which was dodged easily, Deku taking a controlled step sideways to get out of the way. The younger boy kept his momentum and spun into a kick that Katsuki caught above his shoulder, grabbing hold and twisting as he turned around, heaving his own body over his shoulders. Deku fell deftly into a roll and immediately kicked off and tackled Katsuki to the ground. A few fists flew but soon they were wildly slamming knees and elbows into each other, grappling on the cool grass of the field. At some point Katsuki wound up sitting on Deku's hips, pressing them down with his weight and pinning the kid's arms to the ground. Deku jammed his right side upward but Katsuki lifted himself so that Deku's momentum caused him to flip on his side. He slammed his knee into the small of Deku's back, pushing him onto his stomach. He grabbed his own tan wrists and pressed them to Deku's sides. Deku struggled for a moment beneath him before relenting.

"Okay, Kacchan, I give. Let me up," he panted.

Katsuki didn't want to let him up. He wanted to keep him pinned there forever. He was so incredibly aware of Deku's presence in his own body beneath him. The restless feeling in his chest increased.

Katsuki stood and reached a hand down for Deku to grab hold of. He helped him up, they tapped fists again, and then headed back to their dorms.

All the way to the dorm and into his room, Katsuki's feelings of agitation didn't fade. He spent half an hour wearing himself out with push-ups before finally going off to take a shower. The heat seared Deku's skin but he kept it as hot as possible, wanting to kill the beating of his chest. He scrubbed and scrubbed but the feeling wouldn't go away. Back in his dorm room, wearing just a pair of one of Deku's many All Might-themed pajama pants, Katsuki continued to try to fight the feeling the way he fought pretty much any anxious feeling he'd ever felt since he was twelve. The light of his phone lit up his face in the dim room as he searched for something to get off on. He settled on a video of a popular pornstar whose quirk made her skin change color depending on how she felt. This was the fifth time he'd done this as Deku, but the first time he was able to do it in his own room since Kirishima was finally gone. It still took a steeled breath of resolve to reach into his pants and pull out his friend's penis.

Katsuki moved his hand lazily, waiting for the video to pick up, listening through headphones to the soft moans of the pornstar as she was fingered by a guy with a vibration quirk, her skin pulsing with rosy pinks. He forced himself to go faster when they started fucking, not feeling it but desparately wanting to finish and get some sort of release. An absent murmur escaped his throat and his lower abdomen tightened, Deku's voice slinking through the air. He took out one of his earbuds and used his voice again, this time louder and with intent. He revelled in Deku's desperate, breathy moans and tightened his grip, thighs twitching. He dropped his phone onto the mattress. Deku's hand was on his dick and Deku's dick was in his hand. Deku's fingers laid on his abs and he traced his fingers down each of Deku's distinct muscles. "Kacchan," he breathed, closing his eyes as a pleasing heat ripped through his soul. He said the name again, louder and then louder again. He covered his mouth as he came, stifling a groan. Katsuki laid back on his bed, breathing heavily and feeling Deku's semen trickle down the side of his stomach. Dazed, he lay there panting until the heat in his stomach faded.

Katsuki jolted upright on his bed. "Fuck," he muttered, looking down in horror at his semen. "Fuck, fuck, fuck!" he yelled into his hands. Shame burst forth that made him stand and pace his room. Grabbing a dirty t-shirt and wiping his stomach, he lifted a fist to punch his wall but stopped inches short for fear of hurting Deku. Shit, shit, shit, what the fuck did he just do? What the fuck was that? Why did that feel so good? Fucking fucking fuck. He kicked his bed over and over and over before collapsing onto it and shoving his face into the pillow.

He'd fucked up.