Rei Aoki, originally from Tokyo, enters the story at age 17 and ages into her mid-twenties. After Chigusa Tsukikage's acting school dissolves, she moves into an apartment shared with Tsukikage and Maya Kitajima. Rei works as a waitress for income while maintaining ties to Tsukikage's theater work.

Tall with a boyish appearance, Rei is frequently mistaken for male. In the manga and first anime adaptation, she sports short green hair featuring distinctive curled locks framing her face. The second anime redesigns her with simpler, short black hair lacking these curls. Despite visual differences, her androgynous presentation consistently draws admiration from female characters.

Her relationship with Maya is central, evolving into a protective, sisterly dynamic where Rei actively champions Maya's acting ambitions. This support manifests when Rei expresses genuine surprise upon learning Maya received only a doll role in "The Stone's Smile," reflecting her strong belief in Maya's talent. Rei herself performs in the play, demonstrating versatility by portraying both an Italian scoundrel and a refined lady. She consistently advocates for Maya's potential during key career moments.

While specific long-term development arcs across official media remain undocumented, Rei maintains a steady presence within Tsukikage's theater circle throughout the timeline. Her birthdate is December 3rd, aligning with her Tokyo origins.