Nezumi-Otoko (ねずみ男, "Rat Man") is a rodent-like Hanyō and member of the Kitarō Family. He is Kitarō's self-proclaimed "best friend" and a notorious trickster. Born to a Human and a Yōkai, he constantly tries to con members of both races, but almost always gets his comeuppance.

Like Medama-Oyaji, he is an original character created by Shigeru Mizuki.

His nickname is BiBiBi no Nezumi-Otoko (ビビビのねずみ男). The phrase BiBiBi derives from the sound effect binta (slap in the face), but in the 5th anime it is implied to come from the sound Nezumi-Otoko makes as his whiskers tremble whenever he comes across a possible money-making scheme.

https://gegegenokitaro.fandom.com/wiki/Nezumi-Otoko