Shiiba-san no Ura no Kao with Imouto Lip

He's always been the quiet kid at school.He got a secret crush on his classmate named Shiba. She's quiet and mysterious. Never trying to make a fuss, he's barely noticed at all. One day, she cornered him in a vacant classroom and she asked for sex.So why is the prettiest girl in school throwing herself at him...what could the reason be?