Boku dake ga Shitterun Daze

Kaziku Heike transfers to a new high school, where he meets a very cool classmate, Koura-san. Her sister is supposed to be a famous model, but she keeps to herself and she never seems to smile....... But by chance, Kaziku sees Koura-san's extraordinary smile, and his world changes forever! For this smile...... he'll do anything!

(Source: Comic Days, translated)