Kirio is a young boy who serves as a central figure in the conflicts of his secluded community. He is the twin brother of Utao, a discovery that comes as a profound shock to her, as his very existence had been kept a secret from her and most others. Kirio was raised apart from his sibling, taken in and groomed by the rival Hyuuga clan, which sets him on a path directly opposing his biological family. His upbringing was anything but normal; he was subjected to harsh, even cruel treatment from the head of the Hyuuga clan, including physical beatings, and was kept confined in what is described as a cage, a form of imprisonment meant to control and train him.

This abusive background heavily shapes Kirio's personality. He appears eager for combat and displays a wild, almost gleeful enthusiasm when given the chance to fight, which can be interpreted as a release from his otherwise repressive and isolated existence. His outward behavior is marked by a mocking demeanor toward Utao, and he seems to harbor a deep resentment for the comparatively normal and loving childhood she received, including the respect and title of sama she is afforded by their village. Despite his tough exterior and combativeness, he is ultimately a tragic figure, a victim of the power struggles within the village who was weaponized from a young age.

In the narrative, Kirio functions as an antagonist and a rival, most directly to his sister Utao. His primary role is as a Seki, a human with the ability to control a god doll, or Kakashi. His specific partner is the powerful Takemikazuchi, a god doll that possesses formidable melee capabilities and a devastating, semi-instantaneous long-range lightning attack. His power as a Seki is notable; a common belief is that the strength of a Seki is linked to their emotional depth and guilt, and Kirio's tortured past makes him considerably stronger than Utao in their initial encounters. His skill in piloting Takemikazuchi is such that he easily overpowers Utao's Kukuri until she manages to absorb his ultimate attack and destroy his doll in retaliation.

Key to his development is his relationship with Koushiro, a member of the Hyuuga clan who frequently intervenes to protect Kirio from the worst of the clan head's brutality. Koushiro acts as a guardian of sorts, and Kirio seems to have a partnership with him, even accompanying him on missions such as the search for the rogue Seki Aki. While Kirio begins as an opaque and menacing figure, the revelation of his identity and tragic history invites sympathy. His role evolves from a simple enemy to a complicated familial link, and efforts are made by the protagonists to reach out to him, including arranging a meeting between him and Utao where he promises to cease hostilities. However, he remains a largely pitiable and unstable figure, a product of the cycle of abuse and the villages obsessive quest for power, including their secret weapon, the god Amaterasu, which he was originally intended to control.