Susannah Dean’s new, uninvited personality is named Mia. She emerges in Stephen King’s The Dark Tower VI: Song of Susannah as the consciousness of a demon-mother who has entered Susannah’s mind and body.
Mia is obsessed with giving birth to her “chap,” a child created through the strange, supernatural mechanics surrounding Susannah’s pregnancy. As the ka-tet approaches the end of its journey, Mia’s presence becomes increasingly powerful, eventually allowing her to take control of Susannah and travel through the Unfound Door to New York in 1999.
The name can be confusing because Susannah already has a complicated history of divided identities. Before becoming Susannah Dean, she was associated with Odetta Holmes and Detta Walker, personalities that ultimately merged. Mia is different: she is not simply another original facet of Susannah’s personality, but an invading supernatural consciousness with her own motives.
Mia’s story continues into The Dark Tower, where the child she bears becomes central to Roland Deschain’s fate. The official series description calls her a “demon-mother” who usurps Susannah’s body, confirming Mia as the canonical answer.