Nagi Aiza

Nagi is stubborn, sharp-tongued, competitive, and considerably more calculating than her youthful appearance suggests. She frequently dismisses sentimental statements as "cliché" and initially maintains a guarded attitude toward Kousei.

She deeply admires Takeshi. Seeing her older brother's confidence shaken makes her protective of him and contributes to her initial hostility toward the pianist he regards as his greatest rival.

Nagi nevertheless takes music seriously and responds to demanding instruction. Spending time with Kousei gradually allows her to understand him as a musician and person rather than simply the threatening figure at the center of her brother's competitive ambitions.

Nagi is a talented classical pianist who has already received substantial musical training before becoming Kousei's student. Her technical foundations allow him to assign increasingly demanding practice rather than beginning with elementary instruction.

She possesses strong competitive instincts but initially struggles with the emotional pressure surrounding performance. Kousei's teaching pushes her to consider not only whether she can play the notes but what she intends to communicate through them.

Her development culminates in ensemble performance. Playing piano four-hands requires Nagi to coordinate rhythm, dynamics, phrasing, and expression with another pianist rather than treating the instrument solely as an individual competition.

Nagi approaches Hiroko asking to become her student. Her request is partly a pretext: as Takeshi's younger sister, she wants information about Kousei and initially intends to investigate the rival she blames for her brother's problems.

Hiroko sees through the situation and assigns Nagi to Kousei instead. This unexpectedly makes her his first student, forcing both young pianists into unfamiliar roles.

Kousei prepares Nagi for a school festival performance and eventually proposes that they play together. Their demanding four-hand performance becomes both a major step in Nagi's development and a deliberate musical message to Takeshi, helping reconnect him with the competitive drive he had begun to lose.

Nagi's initial resentment gives way to admiration and affection for Kousei as her teacher. Her experience with him also helps her begin defining herself as a pianist separately from her identity as Takeshi's younger sister.

Takeshi Aiza is Nagi's older brother and one of the central influences on her musical life. She admires Takeshi Aiza deeply and becomes distressed when his confidence as a pianist deteriorates.

Kousei Arima becomes Nagi's piano teacher after Hiroko assigns her to him. Although she initially approaches him while concealing her real motive, she gradually develops genuine respect and affection for him through their lessons and performance together.

Hiroko Seto is the pianist Nagi originally approaches for instruction. Hiroko Seto instead recognizes an opportunity for both young musicians and places her under Kousei's guidance.

Koharu Seto is frequently present around Hiroko and Kousei during Nagi's period of instruction.