Kaori Nakaseko is a third-year student at Kitauji High School and a prominent member of the school's concert band, where she serves as the trumpet section leader and also holds the position of band accountant. She is a recurring secondary character in the second season. Physically, Kaori has a gentle and refined appearance, characterized by her short, bobbed dark-brown hair, matching brown-reddish eyes, and a distinctive mole on her left cheek. She stands 166 centimeters tall and is often noted for her elegant and graceful presence, which has earned her the affectionate nickname of the band's Madonna among her peers due to her widespread popularity and approachable nature.

Kaori's personality is defined by her soft-spoken voice, gentle demeanor, and a deep-seated maturity that prioritizes the unity and success of the band above her own personal ambitions. She is well-liked and deeply respected by the other members, particularly attracting the devoted admiration of her underclassman, Yuuko Yoshikawa. Despite her high standing and obvious talent, Kaori is remarkably humble and self-aware, lacking any sense of pretension or entitlement regarding her position as a senior member. Her innate kindness is coupled with a resilient inner strength; she prefers to avoid open conflict but will demonstrate quiet determination when the well-being of her friends or the band is at stake. This is paired with a diligent work ethic, as she is known for her consistent and earnest practice habits.

The central motivation driving Kaori throughout the story is her deep love for playing the trumpet and her desire to see the Kitauji High School concert band succeed as a cohesive unit. This motivation is tested severely during the events leading up to the competitions. As a skilled trumpet player, Kaori has long been considered one of the best in the band, but she had previously suffered under a system that valued seniority over skill. Having endured internal conflicts and bullying from older students during her earlier years, she is determined to prevent the band from fracturing again. Her primary goal becomes ensuring a fair and harmonious environment, even if it comes at a great personal cost. This is most evident when she steps aside to let the prodigiously talented first-year Reina Kousaka take the coveted trumpet solo for the piece Crescent Moon Dance. Although Kaori initially struggles with the disappointment of not being chosen, she ultimately requests a public re-audition not out of ego, but to validate the fairness of the process and put an end to the discord among the band members, accepting the results with grace when Reina proves her superior ability.

Kaori's role in the story is that of a stabilizing and supportive senior figure. She acts as an indispensable pillar of support for the band's leadership. Her closest relationship is with the band's president, Haruka Ogasawara, whom she comforts and encourages during moments of self-doubt, such as when she visits Haruka with sweet potatoes to lift her spirits. She also shares a profound and uniquely close bond with the vice president, Asuka Tanaka. Kaori is often seen spending time with Asuka, including going to cafes, and she has a deep longing to be truly understood by her. This supportive dynamic extends to the underclassmen as well; she is a patient and encouraging mentor, particularly to Yuuko, who idolizes her, and she maintains a cordial relationship with the protagonist, Kumiko Oumae.

Throughout the second season, Kaori undergoes a subtle but significant development arc centered on acceptance and the transition of legacy. Initially, she is depicted as a senior who has quietly shouldered the wounds of past band conflicts. The audition conflict forces her to confront her own limitations and unfulfilled desires head-on. Her decision to concede the solo to Reina marks a pivotal moment of personal growth, where she publicly acknowledges a superior talent and places the band's success above her own pride. This act of selfless maturity not only resolves the immediate tension but also solidifies her role as a respected leader who acts with integrity. Her story arc concludes with a sense of peaceful resolution as she passes the torch to the next generation, satisfied that the band is in capable hands and that she has contributed to a fairer, more competitive environment.

Regarding her abilities, Kaori is an exceptionally skilled and proficient trumpeter, widely recognized as one of the strongest players in the entire band. Her playing style is described as perfect, brilliant, and flawless in its technical execution. While she lacks the raw, powerful expressiveness and natural prodigy of Reina Kousaka, her skill level is never in question; she is a consistent and reliable performer who can play with remarkable precision. Her talent is such that her potential exclusion from a solo role is a significant point of contention among the other band members, who believe she is entitled to the part based on her years of dedication and high level of proficiency.

Beyond her musical talents, Kaori has several hobbies and traits that round out her character. Her favorite color is white, and her hobbies include making sweets and going to cafes, particularly with Asuka. She has been trained since childhood in traditional arts such as the tea ceremony and flower arrangement, which contributes to her refined and elegant image. She has a noted fondness for sweet foods and a dislike for insects and crowded places. After graduating from Kitauji High School, Kaori goes on to attend nursing school, pursuing a career in healthcare.