Manabu is a minor but memorable character in the anime adaptation of Riki-Oh, appearing specifically in the original video animation and related adaptations of the story. He is the son of the prison warden and is depicted as an overweight, spoiled, and immature young man who mirrors his father's cruelty and sense of entitlement. In the manga, he is a largely silent figure who follows his father without uttering a single line of dialogue, serving as a constant, passive companion to the warden. In the animated version, this dynamic is preserved, with Manabu acting as an extension of the warden's authority, though he rarely takes independent action.

Manabu's personality is defined by a childish and sadistic nature. He takes pleasure in the suffering of others and enjoys the spectacle of violence within the prison. His motivations are simple: he seeks entertainment and amusement, often at the expense of the inmates. He is not a schemer or a strategist; rather, he is a passive participant who revels in the cruelty orchestrated by his father. His role in the story is to add to the oppressive atmosphere of the prison and to demonstrate the depth of the warden's corruption, as his son is raised to be just as callous and indifferent to human life.

His key relationship is with his father, the warden, whom he idolizes and follows without question. He has no meaningful interactions with the protagonist Riki-Oh, but he is present during some of the most brutal moments of Riki-Oh's imprisonment. Notably, when the warden orders Riki-Oh to be buried alive, Manabu participates eagerly, throwing a shovel of dirt onto the grave and laughing at Riki-Oh's predicament. This moment captures his lack of empathy and his enjoyment of cruelty.

Manabu undergoes no significant development or change over the course of the story. He remains a static figure, a symbol of inherited evil and the moral decay of the prison system. He has no notable combat abilities or unique skills; his presence is purely narrative, serving to highlight the warden's character and the inhumanity of the prison environment. His notable trait is his appearance: he is large, spoiled, and childish, a stark contrast to the hardened inmates around him. In live-action adaptations, his portrayal is often over-the-top, with a youthful voice that contrasts humorously with his size, emphasizing his pathetic and immature nature. Overall, Manabu is a simple character who exists to reinforce the story's themes of brutality and corruption.