Kōshirō Hyūga is a seki from the Hyūga family, one of the clans that produce individuals capable of controlling ancient wooden dolls known as kakashi. He is the partner of the kakashi named Uwazutsu. Kōshirō is an adult male with black hair and a composed demeanor, often seen carrying out his duties for the family and the village. He is assigned the mission of capturing Aki Kuga, a rogue seki who has escaped from the village's prison and is causing chaos in Tokyo. To accomplish this, he works alongside Kirio Hyūga, a younger member of his family, and later also alongside Mahiru, who is his cousin.

Kōshirō's motivations are rooted in loyalty to the Hyūga clan and the established order of Karakami Village. He does not question his role as a hunter of rogue sekis and approaches his tasks with a serious, level-headed attitude. He is not overtly cruel or abusive, but he is firm and willing to use force when necessary to retrieve Aki. This sense of duty also extends to protecting the village's secrets and maintaining discipline within the clan. When Kirio acts recklessly and is nearly killed by the clan elder Sahei Hyūga for his failure, Kōshirō intervenes to prevent the punishment from turning fatal, showing that he values the lives of his kin even when they have erred.

In the story, Kōshirō first appears in Tokyo when he intercepts a confrontation between Aki and Kyōhei Kuga, using Uwazutsu to drive Aki away. He later battles Aki again, with Uwazutsu tracking Aki by his scent, a notable ability of his kakashi. He also retrieves Kirio after the younger seki's violent clash with Utao Kuga, taking him back to the village. During the return, Kōshirō stands between Sahei and Kirio, physically blocking a lethal strike, which demonstrates his protective nature within the family hierarchy.

Kōshirō's key relationships include his partnership with Kirio, though the two are not particularly close; Kōshirō acts as a handler and supervisor rather than a friend. He also has a connection with Mahiru, his cousin, who is a skilled seki in her own right. His interactions with Aki are adversarial, as Aki is his target, but Kōshirō treats him as a mission objective rather than an enemy to be hated. He does not have direct ties to the main protagonists Kyōhei and Utao, though his actions affect their journey.

Kōshirō's notable abilities stem from his control over Uwazutsu. This kakashi possesses the power to teleport, allowing Kōshirō to appear suddenly in a location and strike from unexpected angles. Additionally, Uwazutsu has an extremely keen sense of smell, which enables it to track a target if Kōshirō has obtained their scent. This tracking capability is used effectively against Aki, who temporarily evades him only by deducing the method. Kōshirō himself does not demonstrate any supernatural physical prowess; his strength lies in his strategic use of his kakashi and his steady temperament under pressure.

Throughout the series, Kōshirō shows little personal development, as he remains a consistent figure fulfilling his assigned role. He does not undergo a change in worldview or abandon his duty. His presence reinforces the weight of the Hyūga family's authority and the inescapable nature of the village's traditions. He serves as an obstacle to Aki's freedom and a reminder that even within the seki system, individuals are bound by family obligations.