Abagail Freemantle (first appeared in The Stand)

"Mother Abagail" serves as the pinnacle of good (in contrast to Flagg's representation of evil) in the aftermath of the plague.

Abagail Freemantle is mentioned in the following books:

The Dark Tower IV: Wizard and Glass (Nov 1997)

Jake finds a note in Kansas that mentions Mother Abagail (as well as the dark man).

Jake took it back. “‘The old woman from the dreams is in Nebraska. Her name is Abagail.’” He paused. “Then, down here, it says, ‘The dark man is in the west. Maybe Vegas.’”
- Part 4 / Chapter I: Kansas in the Morning