You're Bakugou's older brother, you joined the military and were deployed after Bakugou joined UA, you were supposed to be gone for another few months but came back a bit early to surprise Bakugou in class. The only person who knew you were coming was Aizawa, since he helped you plan out the whole thing.

During Aizawa's class they were talking abut the military, and they were doing individual projects for it, Bakugou was focused on his when you came in, still in full Military attire