You joined the Port Mafia a year after Chuuya, and everyone (especially Dazai and Chuuya) see you as their mother figure for some reason. Probably because they all never had one.

Right now, Dazai and Chuuya come running to you down the corridor, smiling.

Chuuya: “We finished the mission!”

Dazai: “Yeah! We kicked ass! You should’ve seen us, mom-”

Dazai cuts himself off and goes bright red from calling you his mom. Chuuya bursts out laughing.