Tatsuo Nagumo is a human student destined to become the vessel for the Chōjin, or Overfiend, an entity prophesied to unite or destroy the human, demon, and beast-man worlds every 3,000 years. Initially shy and lecherous, his ordinary life shatters as supernatural forces converge upon him due to this destiny. After suffering fatal injuries and being declared dead by medical professionals, their souls are absorbed during his resurrection, initiating his transformation. This process awakens latent demonic powers, triggering a physical and psychological metamorphosis into a destructive entity. His relationship with Akemi Itō—innocent in the anime yet sexually adventurous in the manga—results in her becoming the mother of the Chōjin's offspring, binding her to the apocalyptic events his transformation unleashes.

In *Legend of the Demon Womb*, his cousin Takeaki Kiryū is manipulated by the antagonist Münchhausen II to resurrect the ancient evil Kyō-Ō, a rival to the Chōjin. This manipulation forces Takeaki into violent acts, including rape and murder against those close to him, culminating in a fatal confrontation with Megumi Amano. This conflict directly threatens Tatsuo’s role as the Chōjin vessel. Across the saga, Tatsuo’s mutation accelerates, causing catastrophic events like the destruction of Osaka during his transition into the Demon of Destruction. This transformation is a violent, world-altering upheaval eroding his humanity. In the manga, his arc concludes with him decimating the world to enable his offspring's birth, fulfilling the Chōjin’s messianic yet cataclysmic prophecy.

His centrality to the conflict is reinforced through interactions with characters like the Amano siblings. Jyaku Amano is tasked with protecting him, while Megumi Amano initially views him as a savior figure in the anime; her manga portrayal includes antagonism stemming from rivalry with Akemi. Demons such as Suikakujū seek to eliminate him to prevent the worlds' unification, adding layers of external threat to his internal struggle.