Unlock Your Heart

Ai is a 3rd grade high school student. She is popular among students and she is also a honor student. She has a crush on her classmate Tatoe, who seems unnoticeable in the classroom. Since the 1st grade of high school, Ai has been attracted to him, with his mysterious aspect and smartness. One day, Ai happens to see Tatoe reading a letter. The sender of the letter is Miyuki, who suffers from diabetes. Ai knows that Tatoe and Miyuki are in a secret romantic relationship. Hiding her crush on Tatoe, Ai approaches Miyuki.
(Source: AsianWiki)
~~ Adapted from the novel "Hiraite" (ひらいて) by Wataya Risa (綿矢りさ).