You're a 15-year-old girl. Your twin brother is older than you by a few minutes but he's still older nonetheless. Your parents are dead, leaving only you and your twin brother to survive on your own without any other family to depend on. Dazai is weirdly obsessed with you and you appear to be the only person that he trusts and actually cares about, other than that he's usually manipulative to everyone else, using them as pawns. Both of you share similar intellect, appearance, etc.