Keisaku Satō is a fifteen-year-old high school student in Misaki City and a close friend of his classmate, Eita Tanaka. He comes from a wealthy family and lives independently of his parents, which allows his residence to serve as a key meeting place and home base for other characters. His initial encounter with the supernatural elements of his world occurs when the Flame Haze Margery Daw arrives in the city and randomly chooses him and Eita to act as her guides. Following this, they discover a powerful magical map of the city known as the Crystal Altar, which was previously used by a defeated Denizen, and the two friends begin acting as support for Margery in her mission.

In terms of personality, Keisaku is portrayed as generally sociable and cheerful, maintaining friendships with the protagonist Yuji Sakai and others from the beginning of the story. He harbors deep feelings for Margery Daw, though he often hides these emotions, a trait that contrasts with his friend Eita who is more openly expressive. However, when a situation arises where he feels betrayed by Margery, Keisaku displays an outburst of emotion that is even more powerful than what Eita shows, revealing the intensity of his true feelings. This internal conflict and passion are central to his character.

Keisaku's motivations are closely tied to his admiration for Margery and the Flame Haze. He initially desires to become a Flame Haze like Margery and Shana, driven by a wish to fight alongside them. This ambition leads him to view Yuji Sakai as a rival, particularly after learning that Yuji is being personally trained by the powerful Flame Haze Shana and Wilhelmina Carmel. Over time, he comes to accept that he does not need to be exactly like Yuji to be useful. Instead, he changes his goal, deciding to study harder and transfer schools so he can eventually work for the Outlaw, an organization that supports Flame Haze from the human world, thereby assisting Margery in a non-combat capacity.

His role in the story is that of a human ally who provides crucial support to the Flame Haze. He and Eita utilize the Crystal Altar to monitor the city for supernatural threats, acting as an early warning system. As the series progresses, he becomes more directly involved, with his home becoming a shelter and a meeting place for Margery and others. His relationships are primarily defined by his bond with Margery Daw, to whom he ultimately confesses his love, an act that helps rouse her from a state of shock. This romantic relationship deepens significantly, transcending their vast age difference. He also maintains a strong friendship with Eita Tanaka, with whom he works closely as a team, and his relationship with Yuji evolves from rivalry into a more mature acceptance and friendship.

Keisaku undergoes notable character development from a normal high school student who skips class into a dedicated and resourceful supporter of the Flame Haze. He learns to accept his own limitations as an ordinary human and finds a meaningful way to contribute to the fight against the Denizens through knowledge and planning rather than direct combat. As a normal human, Keisaku does not possess any supernatural abilities or powers of his own. His value comes from his strategic use of the Crystal Altar, his willingness to place himself in dangerous situations to provide support, and the emotional anchor he provides for Margery Daw.