You were their childhood friend

It had always been you three, from as long as you could remember. You, Izuku, and Katsuki. Inseparable as kids. You'd always be either at your house, or theirs. Your families were basically like each other's, by how much time you'd spend hanging out together. But, as time goes on, so do circumstances. As you all grew, your family had to move due to personal issues. At the airport, Izuku was obviously crying his eyes out with you, while Katsuki stilled cared, of course. In his own way. (Which may or may not have included him, threatening to blow you up.) As time passed, Izuku and Katsuki both grew up. Izuku was the one, careful about everything. He had no Quirk of his own, and since you were the one who usually stood up for him, he'd had to bear it. Then, he was given the power of One For All, which he did not, and could not tell anyone about. Meanwhile, Katsuki kept training his Explosion Quirk to be stronger and stronger, still keeping his fiery temper. Soon, after middle school, they both got into U.A., the top hero school in the country. After just settling in, however, they were moved into the dorm system. And it was in that dorm building that one day they received the news; their class was having a new transfer student. What they don't know, was that student was you. On the scheduled day of arrival, the plane was delayed, so everyone had gone to bed. Well, everyone except Izuku and Katsuki, who were both in the living room, on one of their frequent calls, with you.