Nezumi-Otoko exists as a half-human, half-yōkai hybrid, instantly recognizable by rodent-like features such as prominent buck teeth and whiskers. Having surpassed 300 years, he maintains a notoriously unhygienic existence, proudly claiming centuries without bathing. This neglect manifests in chronic skin ailments like ringworm and scabies, which paradoxically fuel unconventional combat abilities: knockout-inducing bad breath and potent flatulence usable for propulsion or attack. His physical traits extend to rat-like teeth capable of biting through materials and whiskers possessing iron-like strength, though certain abilities surface situationally and lack consistent documentation. His origins shift across narratives. One version places his mysterious birth as a human infant on an island populated solely by rats. Alternatively, the "Kitarō Jigoku Arc" identifies him as belonging to a species native to the realm between the living and dead, where "Nezumi-Otoko" is a generic term and his true name is "Peke Peke" (meaning "feces" in Tolai). This arc also introduces a mother figure later exposed as the yōkai Sasori-Onna in disguise. He possesses a biological sister, Nezumi-Onna (Rat Woman), and a nephew, Nezumi-Neko (Rat Cat), though these familial connections receive minimal exploration. Greed, self-preservation, and opportunism define Nezumi-Otoko's core personality. He perpetually devises money-making schemes, often posing as a "yōkai researcher," "detective," or Kitarō’s agent to sell fraudulent services. He readily betrays allies—including Kitarō—for profit or to align with stronger forces, yet consistently returns to Kitarō’s side when advantageous. Despite this pattern, flashes of loyalty emerge: he risks his life saving Kitarō from threats like Wanyūdō, assists in cases without payment, and expresses profound grief over lost loves like Caroline. A recurring trait is his intense fear of cats, particularly Neko-Musume. His role and moral alignment fluctuate significantly across anime adaptations. The 1968–1972 series portrays him as overtly deceitful, resorting to effeminate speech when cornered. Infatuation with Yumeko softens him slightly in the 1985 series, leading to greater cooperation with Kitarō. The 1996 iteration emphasizes comedic failure and emotional outbursts, while the 2007 series reduces his schemes and depicts marginally better morals, such as aiding Nurikabe or refusing false testimony against Kitarō. The 2018 version sees him leverage modern technology (e.g., creating a "Yōkai Post" app) while retaining his self-serving nature, notably exploiting kappa labor in a cucumber-based payment scheme. Relationships for Nezumi-Otoko are marked by friction and complexity. His bond with Kitarō oscillates between camaraderie and betrayal, often invoking nicknames like "Kitarō-chan" when seeking aid. He serves as a cynical foil to Kitarō’s altruism, embodying creator Shigeru Mizuki’s satirical intent through his pessimistic commentary on society and human nature. Among other yōkai, he faces distrust yet remains integral to social gatherings like festivals. His unrequited romantic pursuits—targeting figures such as Gama-Reijō, Caroline, or Mana Inuyama—typically conclude in heartbreak or deception.