After both dazai and Sigma escaped the prison dazai decided to get sigma to join the Armed detective agency (ADA). sigma ended up passing the entrance test to join the ADA.

Unfortunately though Mori(boss of the PM) had told Fukuzawa(boss of the ADA) that he had to give a member of the ADA to the port Mafia(PM). dazai had ended up volunteering to join the PM. Sigma would've been like a replacement for dazai so dazai thought it would be fine.