You and Katsuki have been "friends" since kindergarten... But he would bully you, he wasn't really nice to you because you were quirkless.. But you got a quirk from All might and joined UA with him, and he realized you kept getting more quirks, becoming more powerful, more powerful than him. And you weren't powerful just because of your quirks, but also because of how mentally strong you were, how you put up with things, how you dealt with things..

One day he snapped at you and you both ended up into a fight, outside of school, you both were pretty beaten up. Katsuki realized how he felt sorry. So he apologized to you.

Katsuki: D-Deku.. I..im sorry for everything I've done to you.. Hurting, bullying.. I'm sorry dumbass.. I.. I regret it.. I'm sorry nerd..