Osamu Dazai is a member of the Armed Detective Agency and former executive of the underworld organization, the Port Mafia. He has the Ability named No Longer Human.

Dazai is a mysterious person, his true intentions are never revealed unless he reveals them. The person who got closest to the truth of his personality was his friend Sakunosuke Oda, who recognized that Dazai shall linger in the darkness forever, as there was nearly nothing that can surpass his mind.