Kai is a male Tsukumogami, a type of spirit that manifests from objects that have existed for a long time, and is recognized as one of the most powerful individual spirits of his kind. He is known as a suit of armor that came to life, and he holds the distinguished and formidable title of being one of Kyoto’s Three Great Tsukumogami. His existence dates back to the early Edo period, granting him centuries of experience and awareness. Because his immense power was deemed a threat, he was eventually sealed away behind a powerful barrier by another Tsukumogami named Itsuki.

Kai possesses a playful and mischievous personality that often manifests in disconcerting ways. He has a habit of greeting his guests with surprise attacks, suggesting he enjoys putting others in danger for his own entertainment and to test their mettle. This behavior is part of a broader tendency to enjoy playing with people, a pastime he indulges after being sealed for so many years. Despite this dangerous and unpredictable nature, he does not seem overly concerned or malicious when others attempt to surprise him in return, indicating a level of detached amusement. An ancient agreement allows him to request anyone he wishes to be his playmate at any time, a stipulation he uses to summon others and probe them for information. For instance, he has used this right to call upon members of the Saenome, the human exorcists, to learn about the protagonist Hyoma Kunato and his housemate Botan Nagatsuki.

Kai’s primary role in the story is that of a powerful, unpredictable wildcard who tests the resolve and strength of the main characters. He is deeply intrigued by Botan, believing she possesses a dormant potential or power that could shift the delicate balance of power between the human Saenome and the Tsukumogami. This potential is so significant that both sides have forbidden anyone from attempting to use or exploit her, making the investigation into her a sensitive matter. Kai’s interest in Botan naturally extends to Hyoma, who is tasked with protecting her. He deliberately brings up the topic of Hyoma’s traumatic past, specifically the paper umbrella Tsukumogami that killed Hyoma’s siblings, to observe the young man’s determination and emotional state.

Through his confrontation with Hyoma, Kai serves as a catalyst for the protagonist’s character development. In their battle, Kai demonstrates a clear superiority in power and experience, highlighting Hyoma’s inexperience and forcing him to confront his limitations. He bluntly points out that Hyoma’s insistence on using the weapons inherited from his deceased siblings, while an act of honoring them, is actually holding him back from reaching his true potential. Kai suggests that by clinging to the past, Hyoma is preventing himself from developing his own unique strength, a crucial insight into the protagonist’s internal struggle. This revelation pushes Hyoma to realize he must grow beyond his desire for vengeance and his grief.

In terms of abilities, Kai is exceptionally powerful, even among Tsukumogami. While most spirits of his kind can only manipulate the power of the specific object they were born from, Kai’s abilities are far more extensive. He can exert control not only over his own suit of armor but also over related objects, such as full suits of armor, demonstrating a high-ranking and versatile command over ancient weaponry. He has been continuously accumulating energy within his sacred domain in preparation for emergencies, further underscoring his preparedness and immense latent power. His status and power are such that even experienced Saenome like Zohei, Haori, and Kushige are extremely wary of him and do not dare to refuse his demands lightly.