Dazai and Chūya were paired up with each other when they were 15 years old. The man who had paired them was Mori Ougai, the new chief of the mafia, who reasoned that "only diamonds can be polished by diamonds."

The duo was named Soukoku after one memorable night during the events of "Dragon's Head Rush", where they had defeated an enemy organization in a short amount of time that night. This resulted in them recognizing themselves as Soukoku, or Double Black, among other enemy organizations. Th