The Magician and the Glittering Garden

Rio was accepted into a school for magicians, as soon as she arrived she was told to go out and look for a familiar. While out trying to find one, Rio gets lost and runs into a guy named Kyle. Rio ends up liking Kyle because of his kindness. While going back out to look for a familiar, she runs into Arushel who is a part of an ancient demon race. He was running away from the Witch of the West who wanted to claim him as her familiar. Arushel ends up forcing a kiss contract with Rio in order to gain his powers and true form so that he wouldn't have to be with the Witch of the West. But this only enraged her and as punishment for taking away her beloved Arushel, she curses Rio with sorrowful love spell. When Rio is with the one she likes she turns into a guy!

(Source: LuffyNoTomo)