Koya Onihara holds the position of homeroom teacher at the high school where Ayumu Tamari, a student whose body crystallizes during emotional distress, studies. His responsibilities include dedicated support for Ayumu, cultivating a tranquil space within his office to alleviate the student’s anxiety and mitigate crystallization outbreaks.

Koya harbors a personal fascination with crystalline structures, a passion that complements his compassionate approach to Ayumu’s condition. This interest shapes his perspective, allowing him to find aesthetic value in the physical transformations caused by the syndrome rather than interpreting them purely as a medical challenge.

Though he forges a close bond with Ayumu through frequent interactions, he consistently upholds strict professional boundaries, refusing to reciprocate the student’s romantic advances. Their relationship grows strained as his genuine concern for Ayumu’s stability clashes with his unwavering ethical code regarding their roles.

Details about Koya’s past or deeper motivations remain undefined within the documented narrative, which centers on his evolving connection with Ayumu. The story underscores his function as an emotional anchor, navigating the delicate balance between empathy and personal integrity amid the complexities of their student-teacher dynamic.