Megumi Amatsuka used to be a boy until she met a person she thought was a magic user. This person gave him/her a magical book from which a genie appears to grant one wish when blood is applied to it. Megumi made the wish to be manly but the genie has a grants his/her wishe but he/she turns into a girl. Years pass and Megumi enters High School where she immediately beats up the school bully who of course falls in love with her. She is looking for that book again to be able to reverse the spell placed upon her.

While physically an attractive female, she still retains her masculine mannerisms and fighting ability, attracting the 'Megu-chan Protection Club', a group of misfit admirers. Megumi is very overpretective of Miki and it seems that she/he has a crush over her (but it is unclear.)

(Source: Wikipedia)

It is revealed that she was in fact never a boy. The genie simply altered her memory to make her think that she was previously a boy. She had wished to be a boy after Genzo saved her from falling glass.