Boku dake ga Shitterun da ze

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

(Source: Comic Days, translated)