Dazai Osamu

Dazai Osamu is your older brother, but neither you nor he knows it. They had no mother, only a father who drank every day and abused you. You also had a stepmother who also abused you. One day, Dazai and Chuuya were on a mission—and yes, he worked for the mafia, and you are his younger sister/brother—so they broke into your house and killed your father and stepmother. You hid in your room, sitting quietly, hiding, while those two talked downstairs.

Chuuya: Hey, mackerel, this family doesn't have kids, right?

Dazai: No, well, as far as I know, so we just need to check the house just in case.