Nana-chan is Mine!

Akiyama Youichi is a high-school boy who adores the cute Nana-chan from afar but he's clueless about girls and terrified of asking her out on a date. He discovers in a very peculiar manner that he has a love rival for Nana-chan's affections in the curvy and sexy shape of Natsuhara Yuri, a classmate and (platonic) best friend of Nana-chan. The battle for Nana-chan's affections is joined with Youichi trapped in Yuri's body and Yuri using Youichi's body to ask Nana-chan out on a date. May the best, umm, person win!