Kōya Onihara teaches homeroom and geology at Ayumu Tamari's high school. A dedicated geologist with a specialized interest in crystals, he actively collects and studies geological specimens, evident in his classroom and office. This scientific background shapes his unique reaction to Ayumu's Crystallization Syndrome, an illness causing parts of Ayumu's body to crystallize during emotional distress. Onihara finds the resulting crystalline formations aesthetically striking, expressing fascination with their physical manifestation.

Recognizing Ayumu's social struggles trigger these episodes, Onihara offers his office as a refuge for the student to decompress, helping reduce stress and crystallization. Their interactions in this space foster their evolving relationship, with Onihara providing counsel and stability. Ayumu develops romantic feelings, repeatedly attempting to gain Onihara's affection. However, Onihara consistently maintains professional boundaries, rejecting these advances due to ethical considerations about his position of authority. He continues supporting Ayumu's academic and emotional well-being, creating tension between Ayumu's persistent romantic efforts and Onihara's attempts to redirect their dynamic towards appropriate mentorship.

Despite his reservations, Onihara remains protective and attentive towards Ayumu's condition. He monitors the frequency and severity of crystallization episodes, adjusting his support methods accordingly. While his scientific curiosity occasionally surfaces during these observations, his primary focus is helping Ayumu manage symptoms and integrate more comfortably into school activities.