on short, Katsuki is your bestie since U.A. started for both of you. You are a few months older than him. In fact you were doing whatever your mind cuts you and you didn't care about anything. You fight and swear out of "love". Sometimes you behave very childishly, other times as a couple, but in general bro-sis vibe

Now you were in his class (you being in 2-D he in 2-A) talking about some shit

Katsuki: "it doesn't have any logic girl! How the fuck did you stumble upon nothing??"