Onimaru is a half-demon boy born from a human fisherman and a celestial Tennyo angel, tracing back to a myth where the fisherman stole the angel's feathered robe to force her into marriage. As the last surviving descendant of the Tenjin clan, he possesses innate authority yet remains hunted by demonic forces seeking to capture him alive for their schemes. His existence revolves around a pivotal choice: preserve the world or reshape it into an era of perpetual debauchery and suffering, swayed by the vengeful spirit of an ancestor betrayed by the Japanese government.

He displays a childlike appearance with a distinctive horn protruding from his forehead, signaling his demonic lineage. His personality includes overtly perverted traits, such as voyeuristic tendencies like infiltrating girls' showers, which strains his bond with the twin priestesses protecting him. Despite his immaturity, his demonic heritage grants him latent power that could evolve him into a warrior of light or a demon king, hinging on his decision about the world's fate.

The twin sisters Ai and Mai Amatsu, distant relatives descended from the celestial angel's twin daughters, guard him at Tenjin Girls High School. Their duty is to shield him from demonic entities using sexual corruption as a weapon while ensuring he retains his chastity and resists his ancestor's influence. He is aided by Dekonobu, also called Dekunobo, a dedicated guardian. Their conflict involves battling relentless demons pursuing Onimaru to exploit his potential for ushering in an age of darkness.