Yagate Kimi ni Naru: Saeki Sayaka ni Tsuite

The story focuses on Sayaka's past. In the first part, while still in elementary school, Sayaka has her first experience with romantic feelings when she meets a free-spirited girl in her swimming class. The second part explores Sayaka's relationship with her senpai in junior high school and the fallout of its ending, as briefly explained in the manga. (Source: Yagate Kimi ni Naru Wiki)