Tatsuo Nagumo is the destined human vessel for the Chōjin (Overfiend), an entity prophesied to forcibly unite the human, demon, and man-beast realms every three millennia. As a human, he exhibits shyness, lechery, and voyeuristic tendencies. Extreme stress or sexual arousal triggers his transformation into a colossal, destructive demonic form adorned with multiple phallic appendages, embodying both ultimate power and annihilation; one such outburst results in a hospital's destruction. His dynamic with Akemi Itō shifts across adaptations: the manga depicts her as sexually adventurous and pursuing him, while the anime portrays her as innocent, virginal, and a target for corruption whom Nagumo tries to protect. His cousin, Takeaki Kiryū, plays a key role in *Legend of the Demon Womb*. Nagumo's life-saving blood transfusion for Takeaki inadvertently infuses him with the Chōjin's power, allowing the antagonist Münchhausen II to manipulate Takeaki into becoming the vessel for the ancient Kyō-Ō (Evil King), sparking conflict between the corrupted relatives. Nagumo's arc consistently depicts unavoidable corruption and apocalyptic destruction. His final transformation into the Overfiend leads to the decimation of the three worlds, intended to clear the path for a new era of peace. His journey is framed as a tragic inevitability, contrasting his human frailties with the overwhelming destructive force he embodies.