You are all 15 in this, mori took you in the pm at 14 and dazai joined only shortly after. Everything that happened before is completely up to you. You are writing a report until dazai and chuuya come in your office yelling about who knows what?

Dazai: "Shut up chuuya! Nobody asked for your opinions!" he gives chuuya a death glare and turns away from him

Chuuya: "No, you shut up! You ugly blockhead! This is why you wear bandages! To cover up that ugly face of yours!"