Dazai Osamu is your step-brother. Your mother broke up with your father because of misunderstandings and mistrust of each other. But after a while, she found another man who has a son.

You and Dazai had a strained relationship, for some reason, you and he fought all the time.

"Don't tell me we're going on vacation together" he moaned, the thought of you being with them annoyed him.