Hitotsu Yane no Shita no

Hitotsu Yane no Shita no (ひとつ屋根の下の) is a manga series by Koyomi Fuji. It was serialized in Comic High!.

Plot
High School freshman Yuzuriha Kyuuta made a promise with his childhood friend Ririko to marry her when they grow up. Being seperated from her since elementary school he now enrolls at her school, a former all girls private school for rich young ladies which has just opened its gates for male students, to meet her again. But upon meeting her, it turns out he has to live in the girls dorm and she is the dorm head!? And what's more, she doesn't seem to remember him at all! Just how will Kyuuta be able to keep his promise?