Mitsuki Shiina is the popular third-year student council president at protagonist Jun'ichi Nagase's school. She stands at 157 cm tall, possessing mint green hair, purple eyes, and body measurements of 86 cm (bust), 57 cm (waist), and 86 cm (hips), classified as a D cup. Her typical attire is the school uniform, sometimes accessorized with ribbons, a beret, or hair ornaments.

She exudes a confident, relaxed, and enthusiastic personality, frequently engaging in teasing behavior. She particularly enjoys employing Jun'ichi as her "lapdog" for student council tasks or personal favors. Her energetic, sometimes carefree approach contrasts with more reserved characters like student council treasurer Aya Nijo, leading to occasional friction. Mitsuki shows insight in her interactions and occasionally breaks the fourth wall.

Her background and family history remain unexplored. She maintains friendships with multiple characters, including her closest friend and classmate Mikoto Tachibana, alongside Yuuhi Katagiri, Minato Nagase, Tsukasa Kiryu, Nagomi Shiraishi, Karen Ayanokouji, Fuyuhiko Nishino, and Aya Nijo. Her relationship with Jun'ichi is strictly platonic, though she acknowledges his unrequited past feelings.

Mitsuki Shiina experiences no substantive character development or background story across the main series, OVAs, or spin-offs. Her portrayal remains static. In the "Akane Iro ni Somaru Saka Hardcore" OVA, she joins other characters on a beach vacation to Ayanokouji's private island but undergoes no narrative growth, maintaining her consistent personality and role.