Ratio is a recurring supporting character in the anime series Hamatora and its second season, Re: Hamatora. He is a young doctor in his early twenties who practices medicine in Yokohama and, alongside his childhood friend Birthday, helps run an odd-jobs handyman service; the pair is usually treated as a team. In his school years, Ratio studied at the Facultas Academy, the institution associated with people who possess special powers, where his time at the school overlapped with that of Nice and other central figures. He left the academy partway through together with Birthday, and the two lost contact with their former classmates for a period before reuniting with them in the time leading up to the story. This double life defines his place in the series: he attends to patients as a physician while spending much of his energy assisting Birthday with handyman work. He is presented as a composed young man with black hair and blue eyes, and he wears a black eyepatch over his right eye.

In terms of personality, Ratio is serious, reserved, and careful to a fault. He is highly orderly and has a strong aversion to dirt and disorder, and this fastidious side of him becomes a common source of teasing from Birthday, who lives in a much looser and more careless way. Ratio is often exasperated by Birthday's sloppy habits and spontaneous attitude, but his irritation is balanced by genuine concern, and he consistently looks after Birthday and worries about his well-being. Where Birthday is cheerful, easygoing, and focused on enjoying each day, Ratio is the quiet, sensible half of the pair. His composed and dependable manner makes him a stabilizing presence among the more impulsive personalities in his circle of friends and coworkers.

Ratio's motivations are rooted in a sense of duty rather than ambition. He takes his medical work seriously and treats the responsibilities of his profession with care, even as he continues to help Birthday with the odd-jobs business. His loyalty to the people around him is also a strong driving force: when conflicts involving Minimum Holders put his friends in danger, he does not remain on the sidelines and instead takes part in the fighting and decision-making. In Re: Hamatora, which unfolds in the aftermath of earlier events, the renewed crisis surrounding people with powers draws him into direct action alongside the rest of the group, and he continues to offer a steadying influence during the turmoil.

Ratio's notable ability is his Minimum power, which is normally kept sealed and only activates when he removes the eyepatch over his right eye. Once released, the power allows him to see through the surface of another person's body in a way similar to an X-ray, revealing the internal structure beneath. By observing muscle movements and other physical signals, he can read how an opponent is about to move and identify their weak points. After recognizing those vulnerable spots, he closes in and strikes them with precision, usually using brass knuckle-style metal equipment fitted to his hands. His medical training gives him a strong understanding of human anatomy, which makes him especially effective at interpreting the information his power reveals. Because the ability depends on careful observation and exact targeting instead of overwhelming force, it suits his meticulous and disciplined personality well. In the events of Re: Hamatora, he draws on both his combat skill and his medical knowledge when the people close to him are placed under renewed threat.