Keiichi Shimizu, a first-year music student at Seiso Academy, possesses prodigious cello talent. His life revolves around a strict routine of sleeping, waking, practicing cello, and repeating, supplemented by meals, family concert outings, and library visits for music history and instrument research. He lives with his aunt and uncle while his parents, older sister, younger brother, and younger sister reside elsewhere. His older sister regularly sends him costumes, highlighting their close bond.

Physically, Keiichi has light yellow hair, blue eyes, and stands 168 cm tall. Nicknamed "Sleeping Beauty" for his handsome features and constant drowsiness, he frequently naps in random academy locations. His speech is slow, quiet, and unfailingly polite, often punctuated by dazed confusion—like misrecalling his teacher Mr. Kanazawa’s name. He favors simple foods such as onigiri and lists napping and speculation as hobbies.

Musically driven by intense focus and curiosity, Keiichi becomes fascinated by fellow student Kahoko Hino’s emotionally expressive violin style, which starkly contrasts his technical precision. Her melodies persistently occupy his thoughts, inspiring him to explore new dimensions in his performances. Their encounters unfold at concerts or instrument exhibitions, where Keiichi shares artistic vulnerabilities—like his quest for "sheet music" embodying raw emotion—revealed once while playing Bach’s Cello Prelude No. 1 in a Catholic church.

In ensemble activities across official media, including the *secondo passo* special, Keiichi champions collective harmony, comparing cohesive performances to "a woman’s hair beautifully combed." He prioritizes shared emotion over individual skill, advocating for diverse talents like a viola player to enhance their sound. His evolution includes embracing passion-driven music, partly influenced by Kahoko.