What is an antonym of witch?

If you mean a witch in literature, then the perfect set of opposites is the Wicked With of the West and Glenda the Good Witch in "The Wizard of Oz". A more general answer - the opposite of a witch = a fairy god mother
The only antonym for witch could be non-witch. An antonym is a word that means the opposite of another word. There really is no word that means the opposite of witch, unless it is non-witch.