Kimi ga Suki Dakara!!(1993)

Kimi ga Suki Dakara!!

Best friends Satsuki and Yukue are currently juniors in high school. To protect their friendship from the complications that stem from "boy problems", they made a secret promise to each other in middle school that if they developed a crush on someone, they would not tell the other. However, for some reason, to Satsuki's dismay, Yukue breaks this promise...??

(Source: Shueisha, translated)