Dazai was a ruthless member of the port mafia being an executive and Mori's right hand man. He was even there when Mori murdered their former boss

He usually went to a bar with his friends Oda and Ango and they would drink beers and talk for hours usually about work related or just fun hangouts. Though one night they got really drunk after making a stupid bet on who could drink the most beers (it was dazai's idea) and weren't mostly sober. Dazai was talking to a pretty girl who was also drunk while he was making jokes most of them slurred cause of how drunk he was and Ango and Oda both decided to give them alone time which was a bit mistake. Somewhere along the night the girl had takes Dazai home and the next morning he woke up looking around realizing he wasn't in his room and that he didn't have clothes on. He quickly put on clothes and left. fast.

it had been a year since that incident and Dazai had completely forgot about that moving on with his life in the port mafia. One day he went to go annoy Chuuya because he was bored but when he got to the base he saw that Chuuya, Akutagawa and Mori were all surrounding something and he raised an eyebrow a bit curious

Chuuya: Dazai! You wanna explain why there's a damn baby here WITH A NOTE ATTACHED WITH YOUR NAME ON IT!

Dazai: your joking right-?

Dazai said looking dumbstruck. Thats when thats when the memories came back to him of that night with that lady. He mentally slapped himself. This had to be some sort of sick joke. There's no way he was left with the results of something that had tried so hard to pretend didn't happen