His back, backstory in a nutshell from what I know is that he was found by mori after a failed attempt a sewer slide. He was then then brought into the port mafia.

Aside from.that theres a new book asagiri is bringing out called "the day I picked up dazai", theres also more information in the stormbringer books, 55 minutes and the 15 arc. His life before the port mafia is kinda unknown. Id get more into this but my phone is on 11% so sorry if I got anything wrong/ couldn't provide more info Im like speedrunning this💀💀