Hyoi Kageza is the elder of the two brothers who serve as the protagonists of the horror anime Dark Cat. Alongside his younger brother Ryoi, Hyoi is a Dark Cat, a being with the ability to transform between a human appearance and that of a cat. In his animal form, Hyoi takes the shape of a black cat, a color often used to reflect his more reserved and serious personality. The exact nature of his existence is ambiguous, leaving it unclear whether he is a human who was granted supernatural powers or a supernatural being who simply assumes human form.

In terms of personality, Hyoi is the more mature and composed of the two brothers. He maintains a cool and professional demeanor, which stands in direct contrast to his younger brother Ryoi, who is more emotional, naive, and impulsive. This dynamic establishes a classic sibling yin-yang relationship where Hyoi is the steady and strategic elder to Ryoi's more reactive and heartfelt younger brother. Hyoi's personality is also visibly marked by a deep sense of protectiveness toward Ryoi, frequently intervening to rescue him from dangerous situations. His distinctive feline slit pupils are a notable physical trait that hints at his otherworldly nature, even when he is in human form.

Hyoi's primary motivation is to combat evil, specifically a demonic plague of tentacled creatures that preys on the darkness within human hearts. He and Ryoi were originally trained in their supernatural abilities by the demonic entity Jukokubo. However, they eventually turned against their former master and now dedicate themselves to using those same powers to fight him and the evil he represents. Their conflict with Jukokubo is implied to be cyclical and ancient, spanning centuries, and Hyoi seems to fully acknowledge the long-term nature of their struggle. His personal code is one of strategic confrontation and a commitment to neutralizing supernatural threats that exploit human vulnerability.

Within the story, Hyoi and Ryoi operate by disguising themselves as transfer students at a local high school where evil is manifesting. Hyoi also takes the form of a cat to act as a pet for a girl named Aimi Koenji, allowing him to observe the school environment closely. His role is largely that of the more capable and decisive fighter, often taking the lead in combat while Ryoi handles the investigative and emotional interactions with the human characters. In one significant instance, Hyoi bursts into a teacher's office to save Ryoi from a demon-possessed woman who was attempting to corrupt him.

The most significant relationship in Hyoi's life is with his younger brother, Ryoi. He acts as Ryoi's constant guardian and mentor, his actions consistently demonstrating a deep-seated need to keep his brother safe. His other key relationship is with his former master, Jukokubo. This connection is defined by a complex history of tutelage followed by betrayal and an ongoing, seemingly eternal battle. When Jukokubo threatens to return after a defeat, Hyoi responds with a calm and resolute acceptance, stating, "We Will Meet Again," which underscores the enduring nature of their conflict.

The character undergoes limited but significant development. He begins as a capable protector but is humbled when Ryoi suffers mortal injuries in a fight with Jukokubo. In a moment showcasing his resourcefulness and connection to the spiritual world, Hyoi transports the wounded Ryoi to a forest where a mystical white Persian cat, revealed to be the Spirit of the Forest, facilitates his brother's healing. This event highlights his reliance on spiritual allies and reinforces the gravity of the threat they face. His development is not about a change in personality but rather a deepening of his resolve, as he accepts the cyclical nature of his duty to fight evil across the ages.

Hyoi possesses a range of notable supernatural abilities. As a Dark Cat, his primary power is shapeshifting, allowing him to switch between his human and cat forms at will. This transformation is accompanied by a degree of magical materialization, as his clothing appears on his human body after he transforms. His feline form seems to retain his personality and likely some level of intelligence. Hyoi also wields a magical weapon known as the Dark Cat Sword, which has the convenient ability to transform into a concealable pendant when not in use. Beyond physical combat, he demonstrates psychic powers including the ability to use mind control on others and perform minor acts of flight. Furthermore, he has the capacity to perceive and exorcise spirits of the deceased, a skill he uses to help a ghost in the OVA's prologue. These varied abilities make him a formidable opponent against the demonic forces he and his brother are sworn to destroy.