The major characters are the core four: Buffy, Willow, Xander, and Giles. Oh, alright, Cordelia, too, since she was listed in the opening titles from the beginning. But Dave's right and, it never occurred to me before either, Cordelia never did have a significant monster transformation. She was bewitched in "The Witch," and transformed into Nerdy Cordy in "Nightmares," but those don't count.
Faith certainly did some monstrous things and, I suppose one could argue the dark path she chose to walk was her transformation. And, there was the body-swap with Buffy during "This Year's Girl."
Kristine Sutherland never was listed in the main opening titles and, Amber Benson's name appeared only once for "Seeing Red."
Actually, the only other character whose name appeared during the main title sequence that never had a monster alter-ego was Marc Blucas' Captain Cardboard, Riley Finn. He got all hopped up on super-soldier juice, canoodled with vampire hookers, looked a bit rough around the edges, but that was about it.