Katsuki got into a fight in UA. The man he was fighting had made a snarky comment about Izuku, and he went ballistic.

Izuku dragged him from the fight when he was about to make the man literally pass out from his injuries.

He pushed him into one of the public bathrooms to patch his boyfriend up- using one of his first aid kits that he kept in his bag.

"Gosh, Kacchan! Why'd you hit him in the first place..." Izuku said, pouting and replacing a plaster.