Nagi Aiza is a supporting character who appears in the series Your Lie in April. She is a first-year student in the music department of Kurumigaoka Middle School and the younger sister of the pianist Takeshi Aiza. She is often seen wearing her school uniform and has blond pigtails with golden brown eyes. Nagi is initially a piano student of Kosuke Oka but later requests to study under Hiroko Seto. Under Seto's direction, she instead becomes Kousei Arima's first student.

Nagi is introduced as stubborn, childish, and somewhat cunning. She initially approaches Kousei under a false name, presenting herself as a fan of Hiroko Seto, but her ulterior motives are soon revealed. She holds a deep fixation on her older brother Takeshi, whom she once saw as a hero. They were close in childhood, but Takeshi became increasingly absorbed in his rivalry with Kousei, causing the bond between the siblings to thin over time. Nagi's primary motivation for playing the piano is to be noticed by her brother and to bridge the emotional distance between them, rather than performing for glory or perfection. She also wishes to stop her brother's fixation on Kousei.

Nagi maintains a realistic outlook on life and repeatedly expresses her dislike for cliches. She is somewhat arrogant about her piano skills, referring to herself as the Angel of Kurumi Junior High. She also shows a strict sense of propriety, as seen when she corrects others for failing to use appropriate honorifics when addressing Kousei as a senior or teacher. Despite her prickly exterior, she is serious about music and emotionally sincere beneath the surface. Over the course of the story, Nagi warms up to Kousei, eventually coming to comfort him and try to make him smile when he is upset. She develops deep respect for him and admires his piano playing.

Nagi already possessed exceptional piano skills before becoming Kousei's apprentice. She won the Elena International Music Contest and obtained the grand prize at the Matsuno Music Contest. When asked to demonstrate her ability, she easily performed Etude Op. 25 No. 5, the same piece Kousei played at the Maihou Competition. During her lessons, she blames her small hands for limiting her tempo and dynamics. Her skills improve drastically under Kousei's tutelage, allowing her to keep up with his playing during her school festival performance. Her role in the story serves both to provide Kousei with a new teaching experience and to highlight the theme of finding personal meaning in music beyond competition or rivalry.