Nagi is Kousei Arima's first apprentice and student. She is a first year student, studying a music course in Kurumigaoka Junior High School. She initially introduced herself as Hiroko Seto's fan, but her ulterior motives were revealed later on.

Nagi is stubborn and childish. She was introduced as a cunning character who despises Arima Kousei.

Nagi maintains a realistic look in life and repeatedly testifies her hate for cliché. Because of her unerring piano skills, she is slightly arrogant and claims herself as "Angel of Kurumi Junior High". Nagi also appears to have a distaste for anyone who refers to their superiors without the appropriate honorific, as shown when Miike did not refer to Kousei Arima as his Sensei or Senpai, instead addressing him directly as "Arima."

Nagi is often seen to have a brother-complex, seeming to be completely fixated on her brother Takeshi. This complex has driven her to follow in his footsteps in order to get him to notice her and to also stop her brother's own fixation on Kousei.

In the later parts of the anime, Nagi warms up to Kousei, comforting and trying to make him smile while he was upset. Nagi holds deep respect for Kousei and admires him for his piano playing skills. Though it is not stated if she holds affection for him.
Nagi already has exceptional skills in piano before she became Kousei's apprentice. She is the prize winner of the Elena International Music Contest and Obtained the grand prize at the Matsuno Music Contest. When Seto-san asked her to demonstrate her skill, she easily performed Etude Op. 25 No. 5, the same piece Kousei played at the Maihou Competition.

During her lessons with Kousei, she blames her small hands for not being able to play in the right tempo or dynamic. Her skills improves drastically after being tutored by Kousei and can even keep up with his playing during her school festival performance.

Unlike most musicians in the story, Nagi doesn't perform for 'glory or perfection' but for obligation to become closer to her brother.

When she was younger, she saw her elder brother as some sort of hero. They often played together and had a close relationship, but Takeshi started to fix on Kousei Arima. Because of that, he started to practice hard and the strong bond between the two became thinner over the years. In order to be closer to her brother, Nagi starts to play the piano herself and enters a music Middle School
(Source: Shigatsu wa Kimi no Uso Wikia)