Nezumi Otoko is a rodent-like humanoid with prominent whiskers, buck teeth, and skin covered in ringworms and scabs. Standing 160cm tall and weighing 49kg, he typically wears a long brown cloak over a loincloth. He claims never to have bathed, contributing to his repulsive condition, though some accounts show him bathing. His appearance evolved slightly from earlier depictions featuring a different head shape and bangs protruding from his hood. His origins vary across portrayals. Most commonly, he is a hanyō (half-yōkai) born to a human and a yōkai, though his parentage remains unclear. One account states he was mysteriously born as a human baby on an island inhabited solely by rats. An alternative origin in the Jigoku Arc depicts him coming from a dimension between the human and spirit worlds, populated by beings of his kind, revealing "Nezumi-Otoko" as a species designation. In this version, his true name is Pekepeke, derived from a Tolai language term meaning "feces." His mother appears in this arc but is later revealed to be Sasori-Onna in disguise as part of Nurarihyon's scheme. He also has a sister named Nezumi-Onna and a nephew called Nezumi-Neko, though their connections to him are inconsistently portrayed. First appearing unnamed as Dracula IV's servant in the 1960 manga story "The Boarding House," he later became a recurring character. His official introduction occurs when he presents himself to Kitarō and Medama-Oyaji as a "Yōkai Professor" in "The Strange Fellow," though alternative first meetings exist where he steals fish from Neko-Musume, establishing his opportunistic nature early on. His personality centers on greed, cowardice, and self-preservation. He frequently engages in money-making schemes, posing as a researcher, detective, or Kitarō's agent to exploit humans and yōkai. He displays no loyalty beyond convenience, readily betraying allies for financial gain or safety, though he consistently returns to Kitarō's side when advantageous. Underlying loneliness manifests in repeated attempts to find family connections, such as temporarily reforming upon meeting a supposed younger brother (later revealed as a mujina in disguise). He serves as a mood-maker during yōkai festivals despite general distrust from others. His abilities stem from his grotesque physiology: incapacitating putrid breath, flatulence powerful enough for propulsion or flight, and slaps referenced in his nickname "BibiBi no Nezumi-Otoko." He possesses sharp teeth, durable whiskers, and surprising proficiency with modern technology. He claims academic credentials from "Bizarre University's Filth Department" with a degree in "Lazy Studies," though their validity is dubious. His rat-like nature makes him perpetually vulnerable to cat yōkai, especially Neko-Musume. His relationship with Kitarō is complex. He self-identifies as Kitarō's "best friend" despite frequent betrayals, including conspiring with enemies or endangering Kitarō for personal gain. Conversely, instances of genuine cooperation and concern occur, such as aiding Kitarō without compensation or risking his life to save him. Kitarō consistently rescues him despite disapproval. He harbors intense animosity toward Neko-Musume due to their predator-prey dynamic, considering her his natural enemy. His longest-standing positive relationship is with Hakusanbō, known for centuries. Shigeru Mizuki explicitly stated Nezumi Otoko was his favorite character, created to inject social satire and critique into the series. Mizuki used him to express personal philosophies valuing material comfort over altruism, contrasting Kitarō's heroic idealism. The character was partially based on Mizuki's friend Umeda Etaro, known for failed get-rich-quick schemes. Nezumi Otoko narrates Mizuki's "Showa: A History of Japan" series, emphasizing his role as Mizuki's mouthpiece.