Mizore Yoroizuka is a second-year student at Kitauji High School and the only oboe player in the school's concert band. She attended South Middle School, which had a highly prestigious wind ensemble. She is a quiet, introverted, and reserved individual who tends to keep to herself and is often seen practicing alone. Her shyness can sometimes be mistaken for aloofness, but it stems from a deep sensitivity and introspective nature. She experiences strong emotions internally but struggles to express them outwardly, often masking her feelings until they build up and overwhelm her. She has anxiety, which she manages by taking deep breaths, and frequently engages in repetitive actions such as pulling the side of her hair, which can be interpreted as a form of self-soothing.

Her primary motivation is not a love of music itself, but rather a deep attachment to Nozomi Kasaki, a childhood friend who befriended her at a time when she had no other friends. Mizore plays the oboe almost entirely as a way to stay connected to Nozomi, and her playing, though technically perfect, is initially emotionally hollow because it is driven by this dependency rather than personal expression. Her role in the story is central to the first half of the second season, which explores the aftermath of a major schism in the band during the previous year. Mizore was deeply affected by Nozomi's decision to quit the band during that conflict, and she becomes consumed by a fear of abandonment and a desperate need to maintain their connection.

The most important relationship in Mizore's life is her bond with Nozomi Kasaki. She regards Nozomi as the center of her emotional world, and their friendship is characterized by a strong, almost obsessive attachment on Mizore's part. She also forms close friendships with Yuko Yoshikawa, who fiercely protects her, and Natsuki Nakagawa, who helps ease tensions in their social group. Over the course of the season, Mizore undergoes significant development. She initially avoids Nozomi out of fear, but after an unexpected confrontation, they reconcile and restore their friendship. Her breakthrough comes when she realizes that her friendship with Nozomi does not depend solely on playing music together. This realization allows her to play with genuine emotional depth during the Kansai competition, her solo passage helping the team earn a gold medal. In the film Liz and the Blue Bird, she continues to grow, becoming less dependent on Nozomi and deciding to pursue a future in music by attending a music school after graduation.

As an oboist, Mizore possesses outstanding technical skill. She is regarded as the strongest oboist in the history of the school's concert band, with precise pitch control and a unique vibrato technique that an external instructor evaluated as having professional-level potential. She maintains a rigorous practice schedule, arriving early to school every day and practicing for over four hours even during the off-season. She places great emphasis on instrument maintenance. Her performance at the Kansai competition is notable for its emotional transformation, shifting from a technically correct but hollow sound to one that is deeply expressive and connected to her personal growth.