Mitsuru, Karin’s older brother, dedicates himself to supporting his hospitalized sister through relentless work, shouldering the full burden of her medical expenses. His occasional visits aim to uplift her spirits during her prolonged stay, though the toll of overwork leaves him physically drained, culminating in his collapse from exhaustion—an event that shatters Karin’s worldview.

This pivotal moment acts as a catalyst for Karin’s introspection, compelling her to confront the repercussions of resisting surgery and the emotional strain her choices impose on others. His hospitalization steers her toward accepting the procedure, redirecting her focus from personal fears to the collective well-being of those caring for her.

Mitsuru’s background remains sparingly detailed, with no elaboration on his personal life, profession, or relationships beyond his role as Karin’s steadfast sibling. His narrative role emphasizes the silent sacrifices underpinning familial care and the cascading emotional consequences of prolonged health crises on family bonds.