Hideomi Kuromatsu is a character from the series who serves as a supporting antagonist. He is a scientist of considerable standing, having won the Nobel Peace Prize and holding a professorship at Tohto University. These prestigious positions mask his true nature, as he secretly works as the chief scientist for the secret cult known as Gorgom. His role within the organization is to oversee the scientific process of converting humans, whether willing or unwilling, into various mutants to serve Gorgom's purposes. He uses his laboratory at Tohto University as a sanctuary and facility for conducting these horrific experiments and housing mutants.

In terms of personality, Kuromatsu is portrayed as a morally bankrupt individual who exploits his respected social standing to carry out cruel and unethical acts. He is manipulative and predatory in his methods, often targeting vulnerable individuals. In one scheme, he had his henchmen cause car accidents so he could personally treat the injured young men, using their subsequent gratitude to coerce them into joining a terrorist militia for Gorgom. In another instance, he experimented on a friend of the protagonist, Kotaro Minami, injecting him with a hormone derived from a monster to transform him into a werewolf-like creature, callously referring to him as a guinea pig. Kuromatsu is driven by a desire for power and recognition from his monstrous masters, hoping that his successful schemes would impress the Gorgom leadership enough to allow him to be transformed into a mutant himself.

Kuromatsu's primary role in the story is that of a creator, using his scientific expertise to generate threats for Kamen Rider BLACK. He is directly responsible for the surgical procedures that create many of the mutants that the hero must fight. His actions are often personal, targeting those close to Kotaro, such as his friend Toru Hayami. Kuromatsu holds a key relationship with the three High Priests of Gorgom, to whom he answers and whose favor he seeks. He also has a significant but indirect relationship with the villain Bilgenia. After Kuromatsu fails multiple times to defeat Kamen Rider BLACK, the High Priests decide to execute him. However, the Sword Saint Bilgenia intervenes, but not out of mercy. Bilgenia instead kills Kuromatsu off-screen and assumes his appearance as part of a plot to get close to and assassinate Kotaro Minami.

The character's development follows a clear trajectory from a confident and valued operative to a failure marked for death. He is first seen conducting experiments and grand schemes, but as his plans are repeatedly thwarted by Kamen Rider BLACK, he falls out of favor with the Gorgom leadership. His ultimate fate is to be killed for his incompetence, and even his identity is stolen and used as a tool by a more successful villain. His death is confirmed when Kotaro later reads a news report that his body has been found.

Notable abilities possessed by Kuromatsu include his expertise in genetic and biological manipulation, allowing him to perform the "remodeling surgeries" that turn humans into monsters. He is also depicted with mechanical or interchangeable hands, which he uses to conduct his experiments, often generating electricity to power his machinery or perform surgical procedures. His intellect is recognized by the highest levels of human society, making his secret villainy all the more insidious.