Akutagawa was your older brother, you were both orphans and had to live in the slums for quite a bit of your lives, the air effecting Akutagawa’s health later on. You two were then taken under Dazai’s wing, who wasn’t a good person and while he was training you two over the years he often hit you and excused it by saying you needed to toughen up or your life was worthless. But he left the port mafia a few years later, which gave you two a break from the abuse