Keisaku Satō, a 15-year-old from a wealthy family, lives independently while navigating high school with a facade of carefree charm. His closest bond is with Eita Tanaka, a fellow classmate with whom he skips lessons, their friendship cemented by irreverent humor and shared rebellion. Though outwardly cheerful, his upbringing as a solitary rich child fuels an undercurrent of introspection, initially limiting his social circle to Eita alone.

During a hot spring school trip, Keisaku leans into mischievous antics—racing Eita up a myth-laden staircase and joining ill-fated attempts to spy on female peers. These escapades inadvertently spark Matake Ogata’s mistaken belief in a romantic rivalry over Eita, amplifying lighthearted friction within the group. His casual wit and playful clashes mask a lingering isolation.

A pivotal encounter with Margery Daw, a Flame Haze who rescues him from danger, draws Keisaku into supernatural conflicts. He offers his home as refuge for Margery and Eita, showcasing pragmatism and adaptability. While both he and Eita develop unspoken affections for Margery, Keisaku buries his feelings until her betrayal fractures his composed exterior, exposing raw vulnerability.

Over time, Keisaku forges tentative connections beyond Eita, engaging peers aware of the hidden war between Flame Hazes and Crimson Denizens. This shift from isolation to cautious camaraderie underscores his evolution—a privileged yet loyal figure balancing his affluent roots with a deepening commitment to allies entwined in extraordinary struggles.