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Izuku is just trying to get comfortable after classes when the door to his dorm room blasts open.
"C'mon, nerd," Katsuki growls as the door bounces off of the wall. "It's time to—what the hell are you doing?"
"Uh—hello to you too, Kacchan," Izuku says from his bed, though really there's nothing unusual about Katsuki barging in here. Of course, usually he manages to be a little quieter about it; what's the point of sneaking into your boyfriend's room if you let everyone know what you're doing?
Katsuki glares at him, eyes narrowing in contemplation. Stalking over the threshold, he slams the door behind him and clenches his fists. Izuku's first instinct is to yank his blankets up over his body, but, well, he's kind of sitting on them.
Besides, it's too late anyway; Katsuki has obviously noticed the heating pad wrapped around his right arm.
Katsui stares at him for a long moment, and Izuku tries not to shift beneath his gaze. "I know for a fact you didn't break your arm recently," Katsuki finally says ominously, his face darkening as he speaks, and Izuku knows he's already figured it out. "Which means, what? You're still in pain from first year?"
Izuku sighs. "It's not a big deal, Kacchan," he tries. "Recovery girl suggested—it's just that—"
It's just that sometimes, at the end of the day or when the weather changes, his arms ache. It's just that at seventeen, sometimes he feels like his body is betraying him. It's just that there's nothing Recovery Girl can do after the abuse he's put his ligaments through, except to recommend ways for him to manage the pain.
It's just that he didn't really want anyone to know.
"Shut up, Deku," Katsuki bites out, and then he's stomping over to Izuku's bed and yanking away the heating pad. Izuku gives a startled yelp, and Katsuki rolls his eyes. "Move over. Can't believe you were using this inferior shitty thing when you have me."
"What—" Izuku begins, but Katsuki is already crowding into his bed, rolling Izuku onto his left side and curling in behind him. One of Katsuki's arms slings over his waist, the other sliding under his body to wrap around him.
With Katsuki a warm weight against his back and all around him, it takes a second for Izuku to realize that the hand rubbing slowly up and down his right forearm is warmer than it should be. Heat radiates into his skin, seeps into his bones, and the way Katsuki's fingers can curl around his arm makes for a much better fit than the bulk of the heating pad.
There's silence for a moment before Izuku clears his throat.
"You're getting really good at controlling the level of heat you use, Kacchan," he says, because it feels easier to say than anything else. Besides, it's true. His mind is already racing, trying to calculate just how much energy Katsuki is using to keep the heat pouring off his skin so steady. It's the perfect amount of warmth, exactly the temperature Izuku had set the heating pad to.
"Of course I am, nerd," Katsuki grumbles, the slow stroking of his hand never faltering. It feels so good that Izuku thinks his entire body might melt.
He shifts a little, pressing back against Katsuki's chest. Katsuki doesn't protest—just gives his arm a gentle squeeze.
"What was it you wanted, Kacchan?" Izuku asks after a few moments, stifling a yawn. It's early, still, not even time for dinner, but Katsuki is so warm. It would be so easy to fall asleep like this.
"Nothing," Katsuki says. "Just wondered if you wanted to get some training in. Otherwise you're never going to beat me."
"Oh," Izuku says, feeling a spark of guilt. "Sorry, Kacchan. We can—"
"Shut up," Katsuki says again, and then, under his breath: "I like this training better anyway."
Izuku doesn't try to fight away the smile that tugs at his mouth. "Is it just the training you like, Kacchan?" he teases.
"Yeah, nerd. Just the training," Katsuki says. "Now go to sleep."