Kuromaru is an infamous original, hentai-style M.U.G.E.N character. Its base form is that of a giant, floating eyeball of dark skin whose eye turns into a mouth every time it blinks, making it come off as an eldritch abomination. Kuromaru can transform into a variety of objects and/or creatures in order to defeat its opponents, usually by raping them. These forms include a dog, a drill, a muscular humanoid, tentacles, a wooden horse, and much more. Kuromaru also has to ability to clone itself, brainwash its opponents by zonking them with a powerful hypnotic gas, and create portals to either reach its opponent or send them somewhere else.

There are many different versions of Kuromaru throughout the years, including 2, 4, Plus, Press, Lite, etc..

After Kuromaru was released to M.U.G.E.N, some creators started turning M.U.G.E.N into a sexual fighting game by creating or editing dozens of characters, or otherwise converting other characters to be compatible, with hentai moves. Said hentai moves were often derived from Kuromaru's moves like its iconic doggystyle and mating press.

Kuromaru is often associated with the character Delga, even though he does not originate from Delga's home game.