Quirk accident that swaps normal Katsuki with fantasy/barbarian!Katsuki who is absolutely obsessed with following Kirishima around. It's completely normal to barbarian!Katsuki, him and his dragon!Kirishima do everything together after all: bathe, sleep, patch up eachother's wounds, but normal Kirishima gets SOOO flustered over having a somehow even stronger, more rugged version of Katsuki all over him 24/7
Eventually, after seeing Kirishima blush for the 1000th time, barbarian!Katsuki "puts the pieces together" in his mind, dragging Kirishima into one of their rooms before starting to kiss the other man senseless. "Fuck- sorry baby, didn't realize it, you know I can be dense sometimes..." barbarian!Katsuki would murmur between sweet, claiming kisses. It confuses the hell out of Kirishima, but before he can ask anything, barbarian!Katsuki whispers the explanation himself, "You're in heat, aren't you? Middle of your mating season? And gods- your dumbass version of me hasn't even marked you. Probably so needy..." The words make Kirishima absolutely melt, cause they're visceral, carnal, in a way that make Kirishima's everything burn in need. He's not in heat of course, he's just a normal human, but fuck barbarian!Katsuki's words and touch make him feel like a needy animal. And as Kirishima wraps his arms around barbarian!Katsuki, the blonde just shushes him softly, "Don't worry, baby. I'll claim you, I'll breed you, hopefully show your Katsuki how to do it properly when he gets back..."