Chapter Text
Katsuki didn't really like being called "Kacchan" by his younger brother. He was way too old for it now, and his friends always teased him whenever they'd meet on the way back from school. He was walking with his friends, then scowled upon hearing a shrill, familiar voice come from behind him.
"Kacchaaaaan!!"
Shitty hair and Dumb face snickered, mimicking Deku's voice.
Deku bounded over to them.
"Hi, Kacchan!!"
"What the hell do you want, Deku??!!" Katsuki exploded. "And cut that shit out already!! We're too old! You're not five anymore!"
Deku pouted.
"But I've always called you Kacchan..."
Katsuki rolled his eyes, unable to say no to Deku’s stupid pouty face.
"Sup, Izu!!" Sero greeted.
"Ah, hi, Sero-san!!"
Ashido patted Izuku's curls and ruffled his hair.
"Cute as ever, Izuku! Can't believe your older brother's such a gremlin."
Deku laughed at that.
"I'm not a fuckin' gremlin, Ashido!!" Katsuki yammered. Ashido only laughed.
"Yeah! Bakugou's cute in his own way!" Kirishima said, backing him up.
Kaminari furrowed his brows. "Your homosexuality is loud as shit, Kiri."
"Wh-- Denki! I'm just being a good friend!"
"And a bad boyfriend," Kaminari complained.
Izuku giggled. Katsuki's friends were always so fun to be around. It was why he always tried so hard to catch up with them on the way back from school.
And of course, because Katsuki was there. He just loved his brother, as mean as he was. Katsuki wasn't 'nice', but he was good. He was the coolest person Izuku knew.
"Bye, Kacchan~" Kaminari yelled, waving as he and Kirishima broke off from the rest of the group.
"I'll fucking kill you!!"
"You won't," Sero stated, knowing he was all talk.
"Fuck you too, Soy Sauce. Let's go, Deku."
Deku looked over at Sero and Ashido as they split off, waving enthusiastically with a wide smile on his face.
"Bye, Ashido-san!! Sero-san!!"
Ashido blew him a kiss and it made him blush. She was really pretty. Katsuki scoffed.
"Fucking virgin."
Deku blushed deeper.
"Kacchan!"
Katsuki opened the door to their house, yelling a lazy "I'm back" to their parents. The three of them.
Their dad was cooking, while the boys' moms cuddled together on the couch. It was so domestic and wholesome and Katsuki hated it. They were all so gross and loving with each other. It was fuckin’ sickening.
"Hi moms!!" Deku exclaimed, running over to the couch to join them in cuddling.
"Hi, Izuku! How was school? No trouble today, right?"
Deku smiled.
"Nope! Since I went on blockers and cut my hair nobody even suspects anything! I was starting to get worried, honestly. Kept waiting for people to say something, but they didn’t!"
Katsuki rolled his eyes.
"You never have to worry, stupid Deku. If somebody bullies you for it, I'll just beat them up."
Deku beamed.
"Thanks, Kacchan! You're the best."
Katsuki turned pink and started yelling. "I told you to stop calling me that already!!! I'm about to fucking graduate, you little shit!"
Deku puckered his lips like the spoiled brat he was, and Katsuki wanted to slap the freckles off his face. Always fucking using those doe-eyes and pouty lips that made Katsuki want to scream because he could really never say no to Deku, and that was what upset him most. As cold as Katsuki behaved, he really did love his family and it showed.
He never even protested to his parents going out on date nights. They'd usually be gone the whole night, doing fuckshit that Katsuki didn't even want to think about picturing. He and Deku usually choose to have a movie night during these times.
Deku was already thirteen, yet here he was, forcing Katsuki to watch the Princess and the Frog for the nth time. It wasn't a bad movie, just childish, but Deku loved it to pieces. So Katsuki was sitting through it, again, on the opposite end of the couch as his little brother.
"Hey, Onii-chan."
"Fuck do you want, Deku?"
Wait. Did Deku just call him 'Onii-chan'?
Katsuki looked at his brother.
"Is that better? To call you?" he asked.
Katsuki was in shock. The phrase was echoing in his ears, bouncing around in his head. Yeah, it was better than 'Kacchan'. It didn't make him sound kiddie, at his big age. ‘Onii-chan’ sounded good.
"Onii-chan?" Deku asked again.
Katsuki's breath caught in his throat.
"Do whatever you want, nerd," came his response.
Fuck. It was better than Kacchan, it didn't baby him. But it sounded too good. Katsuki liked the way it sounded, or maybe the way Deku said it, way too much. He was thankful for the blanket in his lap because his cock was getting hard and that was bad, that was so bad.
"Okay, Onii-chan!"
Fuck! Katsuki wanted to tell him to switch back to the stupid little patronizing nickname, but he couldn't. It was music to his ears.
And fuel to his dick, apparently. This was wrong on so many accounts. They were brothers.
And Deku was only 13. And they were brothers. And Katsuki was nearly 18. 'Better fuck him while you're still a minor, too,' he thought, fleetingly. No!!!! No! Deku wasn't-- Katsuki did not see his brother like that. That would be gross. His puberty blockers kept him from growing and right now, he looked like a kid. No, not looked like, he was a fucking kid. He barely reached past Katsuki's chest. He was fucking tiny. He was breakable. He was breakable.
Katsuki stood up.
"Deku, I'm going to bed. Don't stay up too late, and wake me up if anything goes wrong."
Deku looked up at him and nodded.
"G'night, Onii-chan."
Katsuki grumbled a good night and went to his room. He sat down on his bed and placed his head in his hands. Why the fuck did that phrase turn him on? Was he…attracted to Deku? He couldn't be. It was Deku for fuck's sake. And they were brothers. Yeah, Katsuki knew he loved him, but that was a given. They were family. He loved every single one of the dumb shits in this household. And Deku was just...he was Katsuki's little brother, it would be weird not to be sweet on him.
Right?
Katsuki jumped at the sound of a knock on his door.
"Onii-chan?"
Katsuki bit his lip. The voice that had usually annoyed him to no end now sounded like music. Music that Katsuki wanted to break down and play his own way. Music he wanted to conduct and shape into an intricate melody of gasps and moans and--
No. Stop.
"Yeah?" came Katsuki's strained response. He cleared his throat.
"'m going to bed now. Can I get a hug? And kiss?"
Katsuki gulped. It was a good thing that he wasn't hard. Thank god for Katsuki's immense willpower. He got up and walked over to his door and opened it to find Deku standing there.
He was holding a teddy bear. At thirteen.
"Aren't you too old for that?" Katsuki asked.
"No. Feels like being hugged," he quipped.
Little shit. What a fucking baby.
"So what's the use of me hugging you then?"
Deku dived into Katsuki's chest and squeezed him in a tight hug. Katsuki laughed and returned the hug, planting a kiss to the top of the boy's head.
Deku's body was so warm and fit so nicely against his own. His skin was so soft, it made Katsuki want to caress his cute milky thighs, and-- No. Fuck. What the fuck was he thinking?
"Okay, that's enough," he ordered, pushing Deku away. "Night, Nerd."
Deku smiled.
"Night!"
Katsuki closed the door quickly, and flopped into his bed. He reached down into his pants to tug at his hardening cock. It was undeniable that he had a kink for that phrase, but that didn't mean he was into Deku. He just liked that phrase. It has nothing to do with his brother.
Absolutely nothing.
At least, that's what he told himself as he stroked his cock. He tried to imagine a girl sucking him off, calling him Onii-chan. She was cute, brunette, just Katsuki's type.
'Onii-chan', he imagined her saying.
Her image faded, much to Katsuki's frustration. But not to worry. It was quickly replaced with the image of his little brother, teary-eyed and blushing, trying his hardest to fit Katsuki's dick in his mouth, doe-eyes concentrated up at him, waiting for something.
Waiting for praise.
Katsuki moaned. Fuck, the image was already there, wasn't it? He was already imagining his little brother with his mouth on his dick, what good was it to ruin the fantasy now?
Katsuki stroked harder and faster as he imagined Deku with saliva dripping down his cute face, a redness tinting his cinnamon-dusted skin as he listened to Katsuki like the good little boy he was. Obeying him as he suckled on his big brother’s cock, getting aroused and reaching down to touch himself.
Fuck, did Deku even know what masturbation was yet?
That was evidently the wrong fucking question to ask, because now Katsuki was imagining himself teaching Deku about his body, instructing him how to play with his virgin cunt, about the pleasures of masturbation, maybe even helping him out by kissing him or sucking on his nipples or fingering him open with his thick fingers, making the kid moan. Deku would probably sound pretty like that. Shit, the thought made his cock throb, precum dribbling out the tip and lubricating his hand.
There was really no turning back at this point. Katsuki was getting off to the thought of fucking his kid brother. He might as well indulge fully.
Mm, fuck. Deku's virgin pussy would probably be tighter than anyone Katsuki's ever had. He wanted to split him open on his fat cock and watch him turn into a little cumslut. Make him cry ‘Onii-chan’ over and over, make him beg for Onii-chan's hot cum. Hngh, Deku would look so sexy bent in half, tears spilling from his eyes from feeling so wrong and so good getting plowed into the mattress by his brother. Shit. Shit! Katsuki couldn’t remember masturbation ever feeling this good, he couldn’t remember the last time he felt so hot, the last time his stomach tensed up in this way. Were fantasies always this effective? Fuck, Deku...! Deku, Deku...!!
Katsuki growled, bucking his hips up into his hand, chasing his release, muttering filth under his breath as he imagined the boy he shared a wall with getting fucked by his Onii-chan. Pleading for more of his cock, wailing for it.
"Fuck, Deku, so good for Onii-chan. Fuck, baby," he muttered, quietly before choking back a moan. He spurted into his hand, cumming hard. Probably harder than he ever had. It was damn near violent, the way he shuddered from his orgasm.
Katsuki was fucked.
//
It was so much harder for Katsuki to be around Deku after that. He shouldn't have indulged. Every fucking time Deku called him Onii-chan he imagined the pre-teen doing things that he probably knew nothing about. It was terrible, but it was... tolerable. Deku was doing what he always did, he was a clingy little shit, and that was both too much and not enough for Katsuki.
Did Deku realize that Katsuki's gaze toward him had changed? Or maybe it hadn't. Maybe Katsuki had always been like this, and maybe that had been why he could never stand the boy while simultaneously being incapable of saying no to him. But it wasn't like Deku ever said no to him either.
Nothing was getting Katsuki off like it used to. He was high as fuck and Sero was coming on to him, but Katsuki wasn't reacting. They usually hooked up whenever Sero got him high alone, switching depending on his mood. Sero was currently trying to give him a hand job, but to no avail
"Dude, I don't think I've ever seen you with limp dick before."
"Shut the fuck up, Soy Sauce. I'm not in the mood, okay?"
Sero laughed. "Okay, now I know something is wrong. You're always horny. You got a new kink or something?"
Katsuki pushed him away. "I don't want you, Hanta."
Sero's eyes widened in shock. Katsuki never used first names.
"Whoa, man. I don't want you either, romantically, but are you okay? You wanna talk about it?"
Katsuki stood up and collected his things.
"No. And I can't talk to you. I'm going home."
Sero threw his hands up in defeat.
"Alright dude, but I'm here if you need me. Oh, and say hi to Izu for me. He's so cute."
Yeah, Katsuki knew. He was pissed that all of Sero's groping hadn't done shit, but just the fucking mention of his brother had his dick twitching.
"Lay a hand on him and I'll fucking murder you," Katsuki warned, glaring at his friend before leaving his room and making his way out.
"Wowww, okay. Totally not what I meant, but love you too, bro!" he said.
Katsuki walked back to his house, quickly, knowing he would have very little alone time before Deku came bounding into his room needing help with homework or asking to play. He usually took naps when he got home for 1-2 hours.
If Deku was sleeping, maybe Katsuki could jerk off to that pretty little face.
Fuck. Katsuki really was hopeless. Did he care anymore? He wanted to fuck him so bad. He practically sprinted the rest of the way home; too much longer in his head and he'd be sporting a boner.
He sighed when he entered the house. Inko was out shopping, Masaru and Mitsuki were at work.
It was pretty quiet, save for the quiet sobs clearly coming from Deku's room.
Shit. Katsuki immediately became upset. Did Deku get bullied again? Had he just been lying so no one would worry? Katsuki felt rage rising up from within him. He was going to kill whoever those little shits were that had made his brother cry. Katsuki set his bag down and started making his way to Deku's room. The poor boy was hiccuping.
"Onii-chan..." Deku hiccuped again.
Katsuki turned the corner and stood in the doorway, about to call out and tell Deku he was here for him. The words got caught in his throat when he laid his eyes on Deku’s image.
He was crying, alright. Just...not in the way that Katsuki had assumed. Deku was wearing one of Katsuki's hoodies, practically drowning in the fabric from how big it was on him. His eyes were shut tight, cheeks flushed a cute pink. One of his hands had fingers teasing his little clit, the other was slowly pushing a dildo in and out of his hole.
"Onii-chan...ahh, Onii-chan...~"
Katsuki had never taken his dick out faster in the entire 17 and a half years of his existence as he had just then.
Fuck, Deku looked so cute and slutty. Katsuki had noticed a few of his clothes had gone missing, but he'd never imagined Deku was using them like this.
The little shit wasn't anywhere near as innocent as he looked. Fuck, how long had he been doing this? Had it been happening every time he decided to hang out with Sero before going home? Katsuki stroked himself, watching as Deku mewled and whimpered, covering the dildo with his slick.
Katsuki stroked faster, biting back his own moans and growls as Deku fell apart in front of him, crying that it was too much, that it was wrong, and still pushing that toy in and out of his tight little pussy that Katsuki was going to pound. Seeing Deku like this, wanting his big brother like this, made Katsuki throw all his fucking morals down the drain. Not that he had a strong attachment to them in the first place.
After all, he always gave Deku what he wanted. And what kind of brother would he be if he didn't?
Deku cried and shook as he was brought to orgasm, the needy mewl of "Onii-chan" bringing Katsuki to his own. As soon as he redressed himself, Katsuki quickly ran back to the entrance, opening and closing the door loudly before yelling an "I'm back" and washing his hands in the kitchen sink. He heard scrambling in Deku's room, frantic mumbling and the sound of drawers opening and closing, probably him trying to get dressed. Katsuki opened the pantry nonchalantly, searching for a snack.
"O-Onii-chan!" Deku exclaimed, looking sheepish, a flush of pink still tinting those freckled cheeks. "You're back early... you're usually gone for another hour or two..."
He looked down at Deku. Did he usually spend the next hour or two masturbating still?
"Yeah, Sero and I didn't fuck today," he said, casually. "So now's the time when you go through my closet and steal my clothes, huh?"
Deku blushed. He'd put pants on but had forgotten to take off Katsuki's sweatshirt.
"Oh! Sorry, I, uh-- they're just so comfy!!"
Katsuki smirked at the obvious lie.
"It's okay, you look cute in them."
Deku's eyes widened and he looked down, embarrassed.
"Th-Thanks, Onii-chan."
"Yeah whatever nerd, just give them back eventually."
Deku nodded.
"You eaten yet?"
Deku shook his head.
"Homework done?"
He shook his head no, again.
"Okay. I'm gonna cook something so you get started on school stuff, alright?"
Deku nodded and went into his room. He grabbed his backpack and brought out his homework, starting on it at the coffee table while Katsuki cooked.
When he was finished cooking, Katsuki brought food over to him and set it down. The blonde was having a hard time not looking like he wanted to eat Deku instead of the food he'd just made. Deku was too concentrated on his homework to notice. Katsuki tapped his head.
"Hey idiot. Eat."
Deku looked up at Katsuki, then down at the bowl in front of him.
"Thanks, Kacchan."
Katsuki bopped him again.
"I told you not to call me that."
Deku nodded. "Right. Sorry, Onii-chan."
Katsuki ate his meal in silence. It was almost funny how obvious Deku's brother complex was, though Katsuki had never known it was this serious. He'd just figured it was because they were close, because they'd always been inseparable, because Deku trusted him with his life and Katsuki would die to protect it. He’d figured it was the kind of attachment he’d grow out of, the kind of attachment that wasn’t...lewd.
Deku finished eating and went back to his homework.
"Onii-chan," he called.
"You need help?"
Deku nodded. Katsuki walked over and sat behind him, hovering over him intentionally, making Deku aware of his presence.
"So what do you need help with?" Katsuki asked, his voice just a tinge more sultry than usual. It was the same voice he used to flirt, but casually. It was obvious from the way Deku stiffened that he picked up on it. But he didn’t address it.
"Um, this one," he said, pointing to problem 5 on the page.
It was simple algebra. Katsuki leaned in closer, explaining to Deku slowly, and clearly, exactly how to solve the problem.
It was math, but Katsuki was making it sound like sex.
He could tell by the redness in Deku's ears that he was definitely uncomfortable. It was fucking cute.
"Did that make sense to you, Deku?" Katsuki asked.
"Y-Yeah," Deku squeaked out. He cleared his throat, then repeated, "Yeah," in a deeper voice.
Katsuki could see Deku rubbing his thighs together.
"You okay?" Katsuki asked, ensuring to coat on a layer of concern to his sensual tone. "You're shaking a little."
"I-I'm fine, Onii-chan."
Deku's voice was so painfully obviously strained, but Katsuki let him think he was able to fool him.
"If you say so."
Katsuki continued to tutor the freckled boy in that same manner until he thought he'd be able to make him cum on command. When Deku seemed so aroused that he looked like he was going to cry, Katsuki left him to go play video games. He left his clothes in the bathroom when he changed, knowing damn well they'd be gone the next time he looked. Within ten minutes, he heard Deku's muffled cries through their shared wall. What a horny little bastard. He smiled to himself, almost moved to tears by how fucking good he was at this. Soon, Katsuki was going to be the only thing on Deku's mind.
Man, Katsuki almost couldn't believe himself. He was probably the only person in the world who could make fucking algebra sound so sexy that people could jack off to it. He was truly gifted. In well, debauchery, maybe, but it was a talent nonetheless, and he was top tier. God, he couldn't wait until the next time their parents went on a date. He'd probably spend the entire night fucking that boy. It might've been bad that Katsuki was looking forward to doing bad things, but all's fair in love and war. And this was both.