GURPS: Red Sky
The wizard who first instructed Thaddeus Solomon’s master, Katherine foolishly tried to trick the Summer Queen in a gambit to gain more power. Exact specifics are unavailable (she’s not talking about it), but the best guess is that she tried to vastly extend her lifespan … and Titania, the Summer Queen, saw to it that she got exactly what she asked for (if not what she wanted.) Now, Lady Katherine is trapped in this not-quite life but is still so terrified of death that she doesn’t want it to end.
Think of those living portraits in the Harry Potter movies. She is still capable of doing amazingly believable illusions (but that’s it – she can’t otherwise affect the real world) so Solomon utilizes her abilities to do a sort of Holodeck for training.
Lady Katherine does not have the breadth of knowledge that Harry Dresden’s Bob possesses and cannot leave the Bavinger House (unless the portrait itself is moved as she’s tied to it). She’s also bitter and sullen most of the time (see before-mentioned ’she’s trapped in a painting!’) though she possesses enough insight to acknowledge that this is really her fault.