Kouzuki Mio is a first-year high school student and kouhai to Kōhei Orihara, recognizable by her short violet hair, blue eyes, and standard sailor uniform accessorized with a hair ribbon and ribbon tie. Mute since childhood, she relies on a sketchbook to communicate, complemented by a naturally cheerful and expressive demeanor conveyed through animated gestures and vivid facial expressions. Her pronounced clumsiness often results in comedic mishaps.

She participates actively in the drama club, where the president designs non-speaking roles to suit her abilities. Mio carries two sketchbooks: one for daily use and the original from her childhood, given by Kōhei when they first met. This notebook represents his broken promise to meet her again and remains deeply meaningful to her.

Their reunion occurs accidentally in high school when Mio trips and spills udon on Kōhei. After insistently offering to clean his clothes, she rekindles their bond and becomes notably attached to him, often seen clinging to his arm.