Kaori Nakaseko has short, bobbed dark-brown hair, brown-reddish eyes, and a mole on her left cheek. She attended Kitauji High School, playing trumpet in the concert band where she rose to become trumpet section leader and served as the band's accountant. Her gentle demeanor and soft-spoken nature made her popular among band members. Kaori enjoys making sweets, visiting cafes with fellow band member Asuka Tanaka, and practicing traditional arts like tea ceremony and flower arrangement, a pursuit since childhood. She dislikes insects and crowds.

During her third year, Kaori competed for a trumpet solo in "Crescent Moon Dance" but withdrew after recognizing Reina Kōsaka's superior skill, prioritizing the band's success over her own ambition. This decision stemmed from her past experience losing solo opportunities to seniority-based selections. Her resignation after the audition revealed a trace of sadness, though she accepted the outcome gracefully. Kaori maintained close relationships with band president Haruka Ogasawara and vice president Asuka Tanaka, spending time with them outside rehearsals. Her interactions with Asuka included lighthearted teasing and deeper conversations about the band's future; Kaori expressed admiration for Asuka's talents and occasionally voiced concerns about the band's hierarchical culture.

Following graduation, Kaori enrolled in nursing school. She later returned as an alumnus to watch the Kitauji Concert Band perform at the Kansai competition during Kumiko Ōmae's second year, demonstrating her ongoing connection to the band. Her character development reflects an understanding of balancing personal aspirations with collective goals and navigating mentorship dynamics within the ensemble.