Nachi is the twin brother of Riki-Oh, born to Hanna in Kokubu Prison as she awaited execution, falsely accused of child murders. Smuggled to safety as infants after Hanna's execution, the twins were raised together until approximately age three, when Riki abandoned Nachi during hide-and-seek to be adopted by Saiga Michiomi. This profound abandonment inflicted lasting psychological trauma, embedding deep resentment toward his brother.

Nachi bears a distinct swastika-shaped birthmark on his right hand, contrasting with Riki's Star of David mark. While narratively unexplained, in-universe speculation links these symbols to theism (swastika) and atheism (Star of David), representing a prophesied collision potentially threatening humanity. Nachi developed supernatural abilities, including a lethal touch and rumored resurrection power, earning him the title "Savior" or "Living God" among followers.

Years later, Nachi surfaced in the lawless territory known as The Cape, ruled by the militaristic cult "God's Judgment" under Washizaki. He became a key figure there, residing within a restricted compound. His first reunion with Riki occurred in a bazaar, where Nachi ignored Riki's attempts at reconciliation. Later, witnessing Riki fight in the underground arena The Temple, Nachi intervened when Riki gravely injured an opponent named Yonabaru—killing Yonabaru rather than healing him, rejecting Riki's pleas for mercy.

Nachi's motivations centered on his unresolved trauma and bitterness festering since childhood. His interactions with Riki were marked by cold detachment and hostility, culminating in confrontations where his volatile powers and emotions positioned him as an antagonist. His journey involved grappling with isolation and the psychological legacy of abandonment, though his character arc remained unresolved.