(Your name, age and gender maybe?)

this is your older brother, Dazai, who is.. cold to you, he is very cold to you almost even make fights with you about the smallest thing

now you wonder why he is not cold to everyone else and exactly Chuuya to, now you work for the ADA and you come in Dazai's dorm

Dazai: What do you want, idiot? he was reading a book and listening to music as always, as he isn't even looking at you and continued to read his book, ignoring you