The Koumori-Kaijin, also referred to simply as Koumori or Bat Mutant, is a bat-themed monster and a senior operative within the evil cult Gorgom in the series Kamen Rider BLACK. It functions as the primary servant and enforcer for the Three High Priests, the organization’s top lieutenants, and carries out their directives with absolute loyalty. Its chief asset is its ability to track and pursue specific individuals, a skill enhanced by natural echolocation that allows it to locate targets over long distances and through obstructed environments. This makes it an expert at reconnaissance and pursuit, constantly deployed to tail the hero or other persons of interest.

In direct combat the creature relies on its physical weaponry, using razor-sharp claws and elongated fangs to rend and bite opponents. The Koumori-Kaijin appears throughout much of the series, from the earliest episodes up to the first theatrical film, making it one of the more persistent and recurring threats among the Gorgom mutants. Its role within the story remains consistent: a dedicated tracker and obedient minion that never wavers in its duty to the High Priests. The monster’s personality is defined by its subservience and single-minded focus on the mission, with little evolution or personal agenda beyond fulfilling its masters’ commands. While the Koumori-Kaijin does not undergo significant development over the course of the narrative, its repeated deployments underline its reliability as a tool of Gorgom’s schemes and a constant obstacle for those who oppose the cult.