Also hailing from New York City, Susannah is an African-American (although as a woman of 1964, she prefers the term "Negro" over the term considered appropriate in Eddie's time, "Black") woman with two major afflictions: her legs below the knees were severed in a subway accident, and a childhood head injury left her with dual personalities, which Eddie incorrectly labels schizophrenia. She is "The Lady of Shadows", the second companion predicted by Walter to be drawn into Roland's world via the mysterious doors.

Initially, her dominant personality is that of Odetta Susannah Holmes, a well-mannered but priggish woman active in the civil rights movement. At times, however, she is taken over by Detta Susannah Walker—murderously psychotic, incredibly crafty, completely unbreakable—whom neither Eddie nor Roland can control. Roland manages to resolve these personalities by forcing her to look through the third door while he himself looks out of it, forcing her to see herself as she really is; thereafter, her personalities are integrated into a single, far more balanced individual, and she becomes Susannah Dean, Eddie's wife. Susannah has access to the memories and personalities of both Holmes and Walker, and can call upon their skills and temperaments at will.

During the drawing of Jake, she submits to sex with the demon of the doorway in order to distract it and afterwards, she becomes pregnant. Susannah shares the experience of her pregnancy with a demon-turned-mortal named Mia, resulting in Susannah housing yet another personality in her mind. Mia takes over Susannah's body (and melds with it, giving her legs for a short while) and runs in it to yet another when of New York, this one circa 1999, to give birth to the demon child.

Susannah is the last member of Roland's ka-tet to leave him (and the only one to survive in a Keystone world), having been given the chance to return to another form of Eddie and Jake in a parallel world.