Hyoi Kagezaki is one of the two titular brothers capable of transforming into cats, assuming the form of a black cat reflective of his personality. Trained alongside his brother Ryoi in supernatural abilities by the demonic entity Jukokubo during their childhood, the brothers later turned against their mentor to combat evil. Their origins remain ambiguous, unclear if they were humans granted powers or supernatural beings assuming human form.

Hyoi maintains a cool, professional demeanor, contrasting his younger brother Ryoi's more emotional and naive nature, establishing a Sibling Yin-Yang dynamic. His distinctive features include feline slit pupils. He wields the magical Dark Cat Sword, which transforms into a concealable pendant. His abilities encompass shapeshifting (with clothing materializing upon transformation), psychic powers such as mind control and minor flight, and the capacity to perceive and exorcise spirits of the deceased.

Hyoi consistently acts protectively towards Ryoi, notably intervening during Ryoi's encounter with the demon-possessed Miss Ogawa. When Ryoi sustains mortal injuries battling Jukokubo, Hyoi transports him to a forest where a mystical white Persian cat, revealed as the Spirit of the Forest, facilitates Ryoi's healing. This highlights Hyoi's reliance on spiritual allies and connection to supernatural realms.

Hyoi acknowledges the cyclical nature of their centuries-long conflict with Jukokubo, responding to the demon's threat of inevitable return with the resolved statement "We Will Meet Again." His actions consistently emphasize strategic confrontation and a commitment to neutralizing supernatural threats targeting human vulnerabilities.