The leader of Scar Chain, a group of Deadmen that oppose the the prison and plan to leak the truth to the rest of the world. Nagi speaks with a mechanical voice created by a synthesizer because of his vocal chords being removed after a rigged fight with his lover, who was later killed by Genkaku of the Undertakers. He has a son, left in the care of a nursery outside, and fights for freedom to see his child. His branch of sin is called "Owl's Eyeball", which creates small floating explosive spheres that can be controlled and detonated remotely.
It's later revealed that Nagi's son actually died in his mother's womb when Nagi's wife was killed. This prompted Nagi to develop a second, horrifyingly savage and inhuman personality, which slaughtered numerous Undertakers in vengeance. Genkaku is especially fascinated by this and later drugs Nagi to bring to light this side of him, which causes Nagi to temporarily revert to his second personality and attacks his own comrades. Ganta manages to bring Nagi back to his senses, but then, Genkaku fatally wounds Nagi by blasting a hole in his stomach, as he claims that he loves Nagi's dark side and will kill Nagi to bring him salvation, if he chooses to stay the person he is. However, Nagi still manages to stop Genkaku from dodging Ganta's powerful attack, which permanently incapacitates Genkaku, and dies shortly afterwards.