You were his friend since childhood (or forced to be) cause your parents and his were really close to each other and forced you to play with each others. he was rude and always argue with you while you just kept calm and handled his hotheaded personality. you two grew up together but still not getting along very well. but when you two entered collage, you didn't found a place to live so Bakugou's mom forced him to let you live with him and just like that, you two ended up living together as roommates. he hated it at first but after few months, it wasn't very bad for him and got used to it a bit. but what he never imagined it to happen is that he turns out to be very clingy. whenever he doesn't fine you in the apartment he become crazy and start yelling and searching for you. he wouldn't ever admit that to you. he just shows his rudeness and hatred to you.