Ahiru no Oujisama

Reiichi is in love with Yumiko, the prettiest girl in school. His problem: he's ugly. When he saves a magical dog from certain death, he is granted one wish in return and he wishes to become beautiful. Now he can be true to his heart, but whenever he touches Yumiko with lustful thoughts, he turns back into his ugly self...

(Source: Anime News Network)