Bungo Stray Dogs, Vol. 2 (light novel)

Before he joined the Armed Detective Agency, Osamu Dazai was the youngest-ever executive with the Port Mafia, the most notorious underground crime syndicate in all of Yokohama. Three of its members Dazai, Odasaku, and Ango formed a camaraderie despite their vast disparities in rank. When Ango suddenly goes missing, Odasaku, a lowly Mafia grunt, receives direct orders from their boss to find him only for Ango s trail to lead Odasaku to a mysterious, foreign criminal organization called Mimic.At last, the reason why Dazai parted ways with the Port Mafia is revealed!