Yukiko Doi appears as a female gymnast and classmate of the main protagonist. She consistently features alongside Satomi Yamazaki within the school's rhythmic gymnastics environment. Together, they function as background competitors who help define the series' athletic setting.

Yukiko and Satomi are depicted as friends of the protagonist, frequently seen cheering her on during training sessions and competitions. Their presence contributes to showcasing the protagonist's journey in the world of rhythmic gymnastics, though they remain secondary figures without dedicated individual story arcs.