Mari Tateishi is a third-year student at Umimaku High School who serves as the president of the school's orchestra club, a large ensemble of over one hundred members. She plays the flute in the wind section. She has soft brown hair usually worn loose beneath her shoulders and light-colored eyes, and she is known for her neat school uniform and sunny smile. Her personality is bright, gentle, intelligent, and kind, qualities that help her manage the substantial responsibilities of leading such a large club. She is well-liked by her peers and is considered attractive. Despite her gentle demeanor, she is determined and works hard to fulfill her duties. Her skill on the flute is not initially at the highest level, and she often receives corrections from the club's demanding conductor and advisor, Hiroaki Ayukawa. Mari's motivations center on her role as club president: she is dedicated to maintaining the club's operations and supporting its members. She takes her leadership role seriously and strives to keep the group cohesive and functioning smoothly. Among her key relationships, she is close friends with Tsubasa Takahashi, the cello part leader, and frequently confides in her. As club president, she interacts regularly with the first-year members including Hajime Aono, Ritsuko Akine, and Nao Saeki, welcoming them into the orchestra. She also works closely with the other student leaders and the advisor. In terms of development, Mari is portrayed as a capable leader who is still growing as a musician. While she is not a virtuoso, she is committed to improving and does not shy away from the guidance of her instructor. Her journey involves learning to balance her responsibilities as a student and club president while continuing to refine her own playing. Notable abilities include her experience in leading and organizing a large orchestra, as well as her proficiency on the flute. She previously served as vice president of her middle school orchestra, indicating a long-standing dedication to orchestral music and leadership.