Tatsuo Nagumo, a shy yet lecherous high school student, habitually spies on female peers, his mundane existence shattered upon becoming the chosen vessel to father the Chōjin—a god-like Overfiend prophesied to unite or annihilate humanity’s triad of realms: human, demon, and man-beast. This role imposes a volatile duality: under extreme arousal or peril, the Overfiend’s power erupts through Tatsuo, triggering violent metamorphoses that slaughter male adversaries and lethally consume women.

The manga amplifies Tatsuo’s perversity, contrasting his sexually adventurous girlfriend Akemi against her anime counterpart’s innocence. Here, his irreversible mutation into the apocalyptic "Demon of Destruction" accelerates the merging of realms, a nihilistic purge to rebirth the Chōjin. The anime introduces Takeaki Kiryū, a cousin weaponized by antagonists to resurrect the Chōjin’s antithesis, framing Tatsuo’s struggles within a war of cosmic manipulation. Akemi’s invulnerability as the Chōjin’s mother temporarily shields Tatsuo, anchoring their doomed romance amid escalating chaos.

Both adaptations chart his tragic metamorphosis from ordinary teen to cataclysmic entity. The manga fixates on his descent into inevitable ruin, climaxing in a lethal clash with demon-manipulated rival Yūichi Niki and final transformation that births the Chōjin, obliterating all worlds. The anime tempers his darker edges, contextualizing his actions within inter-realm conflict.

Throughout, Tatsuo remains a passive instrument of prophecy, his identity eroded by encroaching powers until only cosmic purpose remains—a vessel emptied to forge renewal through annihilation.