Megumi is a nine-year old boy who, despite his age and size, is an excellent martial artist, after saving a sorcerer from getting beat-up by some punks the sorcerer gives him a genie that can grant a single wish.
His wish is to become the "manliest man on Earth," unfortunately the genie is very hard of hearing and misinterprets this as the "womanliest woman on Earth" (in Japanese it’s easier to misinterpret the words involved) and he is transformed into a girl.
Six years later, Megumi has grown into a beautiful girl, the wish having rearranged things so the world has always thought he was a girl, only Megumi and her best friend know how things use to be.
As such Megumi insists on behaves like a guy and is only interested in fighting and martial arts.
Now just starting secondary school Megumi attempts to deal with the day-to-day problems of any other teenage girl.
This long running manga which ended after 20 volumes has also been turned into an Anime series.
English translation of the Japanese manga Tenshi na Konamaiki.