Why cant they just call her a girl, instead of a magician, or a sorceress, or a witch? The truth is, there is one thing that all magicians, sorceresses, and witches all have in common: They study spells, magic, and rituals. Namine does none of that. She's just some girl, who was born with a special gift.
If she were really a magician or a sorceress or whatever, then that would mean that she had learned one day how to alter peoples' memories, just like how magicians and stuff learn spells, and if that were true, then Marluxia wouldn't have needed Namine at all, he could've just learned the spell himself.