Dazai and Chuuya were paired up with each other when they were 15 years old. The man who had paired them, Mori Ougai, the new boss of the mafia, reasoned that "only diamonds can be polished by diamonds" They make up Double Black after they decimated an entire gifted organization in one night, thereafter becoming the "worst nightmare of the underworld". Their affinity in combat primarily lay in Chuuya's physical prowess and Dazai's strategic mind, a feat that was apparently known within the Mafia