Sapphire “Midori” Kawashima is a young teenage girl of below-average height and weight, with a petite build, making her the shortest of the main characters and possibly the entire band. She has messy, wavy, chin-length blonde hair, with a distinctive strand that sticks up from the back of her head. Her eyes are greenish-lime, and she has somewhat thick eyebrows. Sapphire wears a turquoise hair bow with light-green polka dots.

She is usually seen in the standard Kitauji High School uniform for first-year female students.

Sapphire “Midori” Kawashima has a positive and caring personality, always putting her friends before herself. She tends to worry a lot, but her natural cheerfulness shines through. Timid and quiet due to her lack of self-confidence, she is often embarrassed by her own name and prefers to be called Midori (緑). Despite this, she is kind, polite, and very supportive of her friends. Sapphire is also passionate about music, treasuring her double bass, which she has named “George,” and often continues playing it even when her fingers are sore.

Over the years, she grows more confident while maintaining her cheerful and curious nature. She works hard to mentor her juniors, listens and helps when needed, and knows when to temper her curiosity. Sapphire consistently strives to do her best in all that she does.

On her first day at Kitauji High School, Sapphire’s teacher mispronounced her name during roll call, forcing her to correct it with some embarrassment. Afterward, the teacher promised not to make the same mistake again. Soon, she became friends with Kumiko Oumae and Hazuki Katou, forming a close bond as members of the bass section. Sapphire also quickly connected with Asuka Tanaka, whose cheerful and playful personality matched her own, and Asuka admired Sapphire’s skill on the double bass.