Sapphire Kawashima, who prefers the nickname Midori, attends Kitauji High School and plays contrabass in the concert band. She hails from a wealthy background and is an alumna of the elite Seijou Junior High, known for its competitive all-girls concert band program where she learned her instrument.

Midori possesses a petite build, ranking her among the band's shortest members. Her appearance features messy, wavy chin-length blond hair, complete with a distinctive strand sticking up at the back, and greenish-lime eyes. She typically accessorizes her standard Kitauji uniform with a turquoise hair bow adorned by light-green polka dots.

Generally positive and cheerful, Midori nonetheless struggles with self-confidence, leading to timidity and quietness. She feels deep embarrassment over her given name, "Sapphire" (緑輝), due to its unusual reading and frequent mispronunciation, and strongly insists on being called "Midori" (緑), meaning "green". Despite her reserved nature, she is kind, caring, and supportive, often prioritizing her friends' well-being. She forms close bonds with classmates Kumiko Oumae and Hazuki Katou, her first friends at Kitauji, marked by mutual trust. She comforts Hazuki after romantic rejection and confides personal topics in Kumiko.

Musically passionate and dedicated, Midori plays the double bass, affectionately named "George". She demonstrates resilience by practicing even with injured fingers, inspiring less decisive peers like Kumiko. In her second year, she becomes bass section leader, reflecting her growth and the band's recognition. During her third year, she mentors first-year contrabass player Motomu Tsukinaga, guiding him firmly and kindly to ensure good integration and respectful treatment of others.

Outside music, Midori enjoys making sweets, collecting stuffed animals, and visiting prize dispensers for musical instrument charms, especially seeking a "Tuba-kun" charm and expressing disappointment when receiving others. She owns a pet iguana named Macaroon and a giant Japanese salamander plush named Kaiser, bought at the Kyoto Aquarium. She dislikes computers due to her inability to operate them and aspires to attend design school.

Within the concert band's activities, Midori participates in the Colour Guard during marching band performances, a role necessitated by the size of her instrument. Her development shows a shift from a timid newcomer to a responsible leader, evident in her mentorship of Motomu and her consistent support for peers during challenges like the Sunrise Festival and national competition preparations.