You and Katsuki were a thing in Aldera , completely unexpected but somehow it worked out. But it was only for a few months during the last year of middle school. Katsuki being too prideful and stubborn for his own good, he would pick on others, mainly Izuku and you weren’t having it. You thought eventually he’d grow up and get over himself but he didn’t. So you broke up with him in the midst of defending Izuku, giving him yet another reason to despise him. To think his girlfriend dumped him over a quirkless nobody, even if it was his childhood friend

Middle school wasn’t the last he saw of you though. You and the nerd just had to get into UA. He told himself he didn’t care, that you being his ex meant nothing and you were merely an extra he was better than anyway. Still, he always ignored and brushed off the strange feeling in his chest when he saw you. It was only a middle school relationship and those usually didn’t mean anything. But hell, he didn’t get with you for nothing. He didn’t want to admit it but maybe he still carried a possessiveness and protectiveness over you. And over the year, he has to come to terms that you were an equal, and that a part of him still cared for you.

Experiencing and dealing with battles and villains helped him mature and realize his flaws. About time too because that inferiority complex and idolization of himself was getting out of hand. He wanted to be better, to be a better person to be a better hero, to be better than All Might himself. And that was always the plan, but with his own self growth coming to the surface, so did the way he felt about you. He may have been just a brat and a dick back then, but he finally accepted the fact that he never moved on from you.

He knew he was a jerk and that he royally messed up, and once he got over his inferiority complex he knew he needed to make a lot of things up to you, even if you didn’t want to be near him.