Kōsei Arima is a former child prodigy pianist, celebrated as the youngest winner of the Maihou Piano Competition. His perfect pitch enables him to replicate any piece after a single hearing. Trained under the strict and abusive supervision of his mother, Saki Arima, he was drilled in technical perfection. Her death left him psychologically scarred, causing him to perceive his own piano playing as if trapped deep underwater. This condition led him to abandon the piano for two years, during which he lived a withdrawn life and saw the world in monochrome.

He has short black hair and dark blue eyes, usually wearing thick black-rimmed glasses. His typical attire is his junior high uniform: a blue dress shirt, grey trousers, grey blazer, a red-and-white striped tie, and black-and-white sneakers. For competitions, he wears a cobalt blue suit with a royal blue necktie. Casually, he favors polo shirts printed with English quotes and plain trousers.

Introverted, pessimistic, and socially oblivious, Kōsei often remains unaware of interpersonal dynamics, such as his childhood friend Tsubaki Sawabe’s romantic feelings or the rivalries of other pianists. Nevertheless, he is kind-hearted and deeply values his friendships. His childhood was defined by obedience and piano practice, which isolated him from his peers. His mother’s illness and harsh methods deepened this isolation, as he believed perfect performances were needed to please her.

A turning point arrives when he meets Kaori Miyazono, a free-spirited violinist who aggressively recruits him as her accompanist, pulling him back into music. Her emotional, unconventional style contrasts sharply with his mechanical precision, and her influence helps him overcome his trauma. Through her, he begins to see the world in color again and rediscovers an emotional connection to music, aspiring to become what he calls a “weird pianist.”

His relationships shape his growth. His friendships with Tsubaki Sawabe and Ryōta Watari offer foundational support, though he long remains oblivious to Tsubaki’s feelings. His bond with Kaori is most transformative; he develops deep romantic feelings for her, mistakenly believing she loves Watari. He also mentors Nagi Aiza, his rival’s younger sister, guiding her piano studies and earning her respect and affection.

The OVA *Moments* offers additional context through the perspectives of his rivals, Takeshi Aiza and Emi Igawa. It depicts Kōsei’s early career and the impact of his performances on them. Emi was initially inspired by his emotive childhood playing, later frustrated by his shift to metronomic precision. Takeshi admired him as both hero and rival, driven to surpass him. Though Kōsei catalyzed their musical journeys, he remained largely disconnected from their direct rivalry.

After Kaori’s death, he receives a posthumous letter in which she confesses she always loved him and fabricated interest in Watari to draw closer without disrupting his life. This revelation brings closure and renews his purpose. He resolves to continue playing piano in her memory, fully accepting the losses of both his mother and Kaori.

Symbolism surrounds him in the form of cats. A yellow-eyed cat represents his self-doubt and guilt over his mother’s abuse. A blue-eyed cat, encountered with Kaori, symbolizes hope and musical renewal. The death of a blue-eyed cat, hit by a vehicle, foreshadows Kaori’s death and brings despair. Another blue-eyed cat appears after her passing, signifying enduring hope and his resolve to move forward.