Osananajimi wa Onnanoko ni Naare

On one particular day, a high school student, Shuichi, saves a magical fairy Sylphy from drowning in a pond. When Sylphy offers to grant one wish out of gratitude, Shuichi immediately asks for a "cute childhood girlfriend." Sylphy then turns Shuichi's childhood male friend, Iori, into a girl...! The newly female Iori is surprisingly cute...!

(Source: MU)