Niina Shimazaki is a first-year student at Eimei Senior High School and a member of the school's rhythmic gymnastics team. She initially belongs to Class 1-B and later moves to Class 2-A alongside Taiki Inomata, Chinatsu Kano, and her close friend Hina Chono. She has light brown hair that she typically wears up in a bun.

Niina is a kind and observant person who pays close attention to the feelings of those around her. She is particularly attuned to Hina's emotions and is aware of Hina's romantic feelings for Taiki. Rather than interfering, Niina quietly supports Hina, hoping for her friend's happiness. Her supportive nature makes her a dependable presence within their circle.

In her role as a rhythmic gymnast, Niina trains diligently alongside Hina, sharing the same discipline and team. While she is not a central figure in the main drama of the series, she functions as a grounding friend who encourages Hina during stressful periods, such as before qualifying competitions. Her friendship with Hina is characterized by mutual care and concern, and she is genuinely worried when Hina pushes herself too hard.

Niina does not undergo significant personal development over the course of the story; her role remains consistent as a supportive teammate and confidante. She does not possess any standout athletic abilities beyond being a competent member of the rhythmic gymnastics team, and her motivations are primarily tied to being a good friend and contributing to her team.