Ichinose Kai is a piano prodigy raised in a red-light district by his mother Reiko, a prostitute who bore him at fifteen. His father's identity is unknown. From age three, Kai found refuge at an abandoned piano discovered in a nearby forest. Despite its deteriorated state, he cultivated an extraordinary ability to produce sound where others could not, learning music instinctively by ear and composing original melodies. This self-taught foundation defined his early style, emphasizing raw emotional expression over technical precision.

Kai possesses an androgynous appearance, often mistaken for a girl, characterized by light brown ruffled hair and brown eyes. His personality is outspoken and fiercely independent; he openly challenges authority and physically confronts those insulting his mother or background. Despite societal stigma, he exhibits no shame about his upbringing and maintains a close bond with Reiko, who supports his musical pursuits. His defiance extends to resisting formal piano lessons, preferring the freedom of the forest piano.

Key relationships shape his journey. He befriends Shuhei Amamiya, a classmate from a prestigious musical family, after Shuhei witnesses him playing the forest piano. Their dynamic evolves into a friendly rivalry, occasionally strained by Shuhei's envy. Sōsuke Ajino, a former concert pianist turned teacher who once owned the forest piano, becomes Kai's mentor upon recognizing his talent. Ajino guides Kai's technical development, pushing him toward competitions despite Kai's aversion to rules and etiquette, helping him navigate the classical music world's rigid structures.

Kai's early competition entry ends in disqualification due to poor etiquette, not skill, highlighting his initial disconnect from formal expectations. A pivotal loss occurs when the forest piano, eroded by weather, is destroyed in a lightning-caused fire. This plunges Kai into despair, but he rediscovers purpose performing with a street band. During this period, he cross-dresses as "Maria" to work incognito at Club P, a strip club, avoiding competition restrictions while continuing to play. This experience reinforces his adaptability and dedication to music as personal expression over conformity.

His growth culminates in the International Chopin Competition. Initially insecure amid technically polished rivals, he struggles with comparisons, particularly to Shuhei. Ultimately, he embraces his unique connection to music, channeling memories of the forest piano during his performances. This authenticity resonates with audiences and judges, securing his victory. The win symbolizes his reconciliation of innate talent with refined skill, affirming his belief that music transcends competition and reflects personal truth. Post-competition, he continues evolving as a pianist, balancing artistic integrity with professional demands.