Nagi Kengamine, also known by his alias Owl, is a Deadman who served as the leader of Scar Chain, a rebel group formed by inmates who opposed the corrupt administration of the prison. Before founding Scar Chain, Nagi was held in G Ward where he had a relationship with a female Deadman. When the two were forced to fight each other in the Carnival Corpse death matches, Nagi intentionally lost in order to avoid harming his partner, who was pregnant at the time. His vocal cords were removed after that rigged fight, forcing him to speak through a mechanical voice synthesizer. His lover was later killed by Genkaku, a member of the Undertakers. Nagi believed he had a son who was being cared for outside the prison, and he fought for freedom primarily to be reunited with that child. It was later revealed that his son had actually died in the womb when his mother was killed, a truth that shattered Nagi upon recollection.

Personality-wise, Nagi is a composed, thoughtful, and determined individual. He acts as a calm and rational leader, able to earn the trust of his fellow Deadmen. His primary motivation is to expose the prison’s atrocities to the outside world and to secure a future for his child. However, the trauma of losing his family triggers a separate, savage, and nearly mindless second personality, driven solely by vengeance. This alter ego slaughtered numerous Undertakers and is especially feared. Genkaku becomes fascinated by this darker side and later drugs Nagi to bring it out, causing Nagi to temporarily attack his own comrades. Ganta Igarashi, the series’ protagonist, manages to bring Nagi back to his normal self, but Genkaku fatally wounds Nagi by blasting a hole in his stomach. In his final moments, Nagi uses his remaining strength to help Ganta land a decisive attack that permanently incapacitates Genkaku.

Nagi’s Branch of Sin is called Owl’s Eyeball, which allows him to create small, floating spheres of blood that can be controlled remotely and detonated as explosives. He can also manipulate the blood within his severed limbs, enabling trick attacks such as throwing a dismembered arm to generate explosives behind an opponent. His abilities, combined with his leadership and resolve, make him a formidable figure in the prison’s resistance movement. His key relationship with Ganta is one of mutual respect and guidance, as Nagi mentors the younger Deadman and entrusts him with the mission to continue fighting for truth. Nagi’s development arc moves from a hopeful father fighting for a child to a man broken by the revelation of his loss, ultimately choosing sacrifice to help his allies and take down his tormentor.