Nezumi Otoko, also known as the Rat Man, is a rodent-like half-human and half-yōkai hybrid who serves as a recurring character in the Gegege no Kitarō series. He is the self-proclaimed best friend of Kitarō, though his loyalty is consistently unreliable. His origins are ambiguous: he is typically described as the child of a human and a yōkai, and he has lived for over three hundred years. In that time, he has almost never bathed, leaving him perpetually filthy, foul-smelling, and covered in welts and sores. He wears a long, hooded robe and has a rat-like face with whiskers and prominent front teeth.

His personality is defined by greed, self-interest, and a constant pursuit of money and women. He is a notorious trickster who will betray anyone, including Kitarō, if he believes there is profit to be made or if a stronger enemy appears. He often cooks up get-rich-quick schemes, which almost always fail and result in his humiliation or punishment. Despite this, he is not a pure villain. He occasionally shows genuine friendship, compassion, or a sense of honor, especially when children are involved or when he realizes he has gone too far. He has a nihilistic streak and sometimes voices cynical observations about society, reflecting the real-world philosophy of his creator. His motivations stem from a deep-seated insecurity: as a half-breed, he has been rejected by both humans and yōkai, leading him to prioritize self-preservation and material gain as a means of survival.

In the story, Nezumi Otoko occupies a fluid role, often switching between antagonist and ally. He frequently causes trouble by releasing sealed yōkai, aiding villains, or instigating conflicts for personal gain. However, when the situation becomes dire, he may switch sides and help Kitarō, sometimes at great personal risk. His actions often serve as a source of comic relief, but they also carry social satire, as he embodies human greed and folly. He is a fixture of the series, appearing in virtually every adaptation.

His key relationships revolve around Kitarō, with whom he shares a deeply ambivalent bond. Kitarō repeatedly forgives his betrayals and considers him a friend, while Nezumi Otoko himself, despite his selfishness, has risked his life for Kitarō on several occasions. He is despised by most other yōkai, particularly Neko-Musume, who is his natural enemy due to the cat-rat dynamic, and Medama-Oyaji, whom he has mistreated. The creator Shigeru Mizuki stated that Nezumi Otoko was his favorite character and the one who best expressed his own cynical worldview.

Over the course of the various anime series and manga, Nezumi Otoko's characterization has evolved. In earlier adaptations, he was depicted as more malicious and willing to commit serious harm, but later series softened him into a more comedic and occasionally sympathetic figure. He sometimes demonstrates personal growth, such as taking an honest job or showing genuine care for others, though he always reverts to his old ways.

In terms of abilities, Nezumi Otoko lacks significant supernatural power. His primary weapons are his extreme filth and odor. His breath and body odor are potent enough to incapacitate humans and yōkai alike. He can also release powerful flatulence, which he can weaponize as a chemical attack or even use as propulsion to fly. He is cunning and persuasive, often talking his way out of trouble or manipulating others. While generally ineffective in direct combat, his repulsive hygiene makes him a surprisingly dangerous opponent.