The fairy comes from the celtic tradition. In the 12th century novels, though, she's assimilated by the Christian ideology (check the Chrétien de Troyes novels.) She remains a disturbing character, living beyond a misty lake, not really human and not divine either. She can be deceiving: Viviane steals Merlin's secrets after he fell in love with her; that's the origin of her powers.

The masculine counterpart of the fairy is the wizard. He's assimilated to the mad and can live, like Merlin, in the woods. Forests are fightening places for the medieval man: there live beasts and supernatural beings.