Mai is the vessel of the Nameless Grimoire, which allows her to read people's thoughts or memories or obtain information about objects by tasting them. Her true name is Mai Hazuki and she's the heir of the Hazuki Family, one of the twelve noble families that govern the world. Shortly prior to the events of Remix Heart, the Nameless Grimoire inadvertently fused with her body and transformed Mai from a boy into girl. She was not able to reverse the transformation and her father sent her to the Torifune Military Academy under an alias. While Mai initially wanted to return to her previous form, she eventually became female in both body and mind, came to accept her new form and now considers herself to be a woman.
Mai was given a powerful magic spear called Outseal by Kajun in Variable Heart, that she used ever since as her preferred weapon.
As Mai's own media almost always portrays her in female form, she only warrants the genderswap_(mtf) tag for pics that include her male form or otherwise allude to her originally being male. See topic #11614 for details.