Nezumi otoko is a half-human, half-yōkai creature who serves as a foil and occasional ally to the protagonist Kitarō. He has a long, oblong head with a face that evokes a rodent, complete with prominent front teeth and whiskers, and he typically wears a single tattered robe-like garment wrapped around his body. He is known by the nickname Bibibi no Nezumi otoko, a play on Kitarō's own Gegege moniker, and his true name has been given as Nezumi Pekepeke in certain storylines. His origins vary across versions, but he is most consistently described as being born from the union of a human and a yōkai, or, in some accounts, as a human baby mysteriously born on an island inhabited only by rats. He is believed to be over three hundred years old and is notoriously unhygienic, rarely bathing and carrying a powerful, repulsive odor.

In terms of personality, Nezumi otoko is defined by greed, self-interest, and a complete lack of moral scruples when it comes to making money or securing his own safety. He is a compulsive schemer, constantly devising get-rich-quick plots that typically involve fraud, theft, or selling out anyone nearby, including Kitarō. He is a coward at heart and will switch allegiances instantly if he believes the stronger side will benefit him, often betraying his friends only to crawl back when the tide turns. Despite these flaws, he is not a purely evil character. He possesses a certain roguish charm and, on rare occasions, acts out of genuine loyalty or affection, usually when his friends are in mortal danger. He also craves companionship and romantic love, though his schemes and repulsive nature almost always sabotage these desires. His motivations are almost entirely materialistic, but he occasionally reveals a sentimental side, remembering small kindnesses done to him when he was destitute.

His role in the story is that of a trickster and a disruptive element. He is the source of much of the conflict and comedy, jumping between the roles of antagonist, nuisance, and reluctant helper. He is neither a true hero nor a true villain, occupying a gray zone that creates narrative tension. He frequently causes trouble that forces Kitarō to intervene, and his schemes often backfire spectacularly, leading to his own humiliation or punishment. Nonetheless, when the situation is dire, he can be counted on to contribute in his own grimy way, and he has saved Kitarō on multiple occasions through quick thinking or by swallowing his pride.

His key relationships revolve around his connection with Kitarō. The two are described as having a classic love-hate, bad-friendship dynamic. Nezumi otoko constantly uses Kitarō, betrays him, and tries to profit from his exploits, yet he also shows genuine distress when Kitarō is in danger and has wept at the thought of losing him. Kitarō, for his part, is endlessly exasperated by Nezumi otoko but never completely abandons him, often stating that he is a scoundrel but not a true villain. The character Neko Musume, a cat yōkai, is Nezumi otoko's natural enemy, and he is terrified of her due to his rat-like nature. He also interacts with other regulars such as Medama Oyaji, Sunakake Baba, and Konaki Jijī, who generally tolerate him with varying degrees of annoyance.

Across different anime and manga series, Nezumi otoko has shown development, though his core nature remains constant. Early portrayals sometimes highlighted his friendship with Kitarō more strongly, while later series emphasized his cowardice and greed. In the sixth anime series, he was given moments of genuine emotional weight, including a scene where he helped rouse Kitarō from despair after the loss of allies. His character arc typically involves a cycle of scheming, failure, brief repentance, and relapse, reflecting a static but entertaining personality that the creator, Shigeru Mizuki, described as essential to the series, as his chaos drives the story.

His notable abilities are unconventional and tied directly to his filth. He has a foul breath that can knock opponents unconscious, and he can generate powerful flatulence that functions as a jet propulsion, even allowing him to achieve limited flight by spreading his cloak. His body is covered in ringworm and scabies, and he can consume rotten or spoiled food without ill effect, storing gas in his stomach for later use. His sharp teeth can bite through tough materials, and his whiskers have been depicted as strong as iron wire. He is also quick to slap opponents, a move that gave rise to his nickname. While he is weak in direct combat and is notably afraid of cats and cat-related yōkai, his resourcefulness, knowledge of the yōkai underworld, and ability to talk his way into and out of situations make him a survivor. He is also a competent liar and con artist, able to adopt various false identities and titles to further his schemes.