Student or Learner
Mrs. Casnoff looked up at me like she’d forgotten I was even in the room. “No demon could escape from that particular pond,” she answered in her velvety voice. “There are protection spells in it. It . . . holds anything it doesn’t recognize as a witch, faerie, or shifter.”
It's a shortened form of "shapeshifter".
This is some one who can magically change (shift) their physical shape or appearance - probably werewolves given the context! :)