Hisaya Miki is a student at Saint Ishiyama High School and a member of the Rokkisei, the group of powerful fighters known as the Six Holy Knights. He is notable for being the only first-year student to hold a position in this elite group.

Miki has a history with the protagonist, Tatsumi Oga, and his friend Takayuki Furuichi, as the three attended the same middle school. During that time, Miki looked up to Oga's immense strength and considered him a friend. An incident with a local gang profoundly shaped his life after he forgot a charm at an abandoned factory and returned to find it, only to be severely beaten by the gang that used the factory as a hideout. Oga came looking for him and began fighting the thugs. Miki tried to confront the gang leader himself but was easily overpowered, receiving the distinctive long scars on his chin from the leader's sharp nails. Despite his injuries, Miki was grateful to Oga for standing up for him, which solidified his feeling of being Oga's true friend.

The day before Miki was scheduled to move away to Nara, the same gang came to their school looking for Oga. Miki rushed to help his friend, but Oga instead attacked him, beating him up and calling him pathetic, which planted a deep-seated hatred in Miki's heart. When he eventually returned and enrolled at Saint Ishiyama High School, he begged the powerful martial artist Kaname Izuma to teach him, driven solely by the desire to become strong enough to defeat Oga.

Miki is a short teenager with black or purple hair and dark eyes. He is easily recognized by the two scars on his left chin and a beauty mark under each eye. His personality was originally gentle and admiring of strength, but his perception of Oga's betrayal warped his character. He became deeply resentful and obsessed with surpassing his former friend, even rudely snubbing Furuichi upon their reunion, claiming to be allergic to delinquents. Miki possesses a strong sense of pride, desiring to prove his power legitimately without cheating, and he demonstrates considerable determination in his goals.

As the story progresses, the misunderstanding that fuelled his hatred is resolved. When the same gang leader from his past reappears seeking revenge on Oga, Miki learns the truth: Oga had beaten him up on that day before his move to Nara specifically to make him seem pathetic and unworthy of the gang's attention, thereby protecting him from their wrath. Overwhelmed with remorse for his years of hatred, Miki attempts to rekindle his friendship with Oga, though their interactions become less frequent afterward.

In terms of abilities, Miki is a highly skilled martial artist. He underwent rigorous training under Kaname Izuma, mastering the fictional Izuma-style martial arts, and is also a master of Hakkyokuken. He serves as the captain of the karate club at Saint Ishiyama. His fighting style relies heavily on his exceptional agility, which allows him to evade attacks and even finish a fight with Oga without taking damage. One of his powerful techniques is the Izuma style finisher known as the Black Owl Killer. While strong enough to defeat opponents like Hajime Kanzaki with a single punch, his attacks are less effective against exceptionally powerful fighters like Oga or Hidetora Tojo. Miki also appears to have some inexperience when dealing with multiple opponents simultaneously. His resilience is not particularly high, but his martial arts prowess is notable enough that he is one of the few humans capable of sensing demonic power.