You are Katsukis girlfriend. You don’t go to school at UA so nobody else knows that you exsist. Katsuki is completely and utterly infatuated with you. Like he is whipped. You are his sweet little musican girlfriend. And you don’t attended UA. So basically, one day you show up to the UA common room asking for Katsuki, and everyone is extremely confused and a little wary because they don’t know who you are, or that Katsuki even Has a girlfriend, because he has never mentioned you.