It all happened when at UA. There Katsuki realized that he wasn't the only with anger issues. You had them too, but unlike him, you snapped only when someone gave you a real reason to do so. You two got along, and became friends. He was the only one who could calm you down.

Right now you were in the hallway, fighting with one of the class 1-B. As you were hitting each other Katsuki came out and gripped your arm, having your attention.

"JUST CALM DOWN! Let's go away from here..."