Easy to find when you know where to look, but she's nowhere near Thornwood Watch. Actually in a large crypt behind O'Daxxia main city.

The screenshots tab and map is accurate. Find Burt Bedmountain in north east O'Draxxia city. On the other side of the wall, outside through the window you'll see a large statue of a gargoyle. Behind the big statue there is a doorway up a short flight of steps. Head through the doorway and keep going and you'll find her near the end of the passageway.

If you keep going past Carolia there's a large statue at the end with a book that gives the "Ahib Myth: The Book of Thornwood" knowledge for 100 Energy.