Rosemary, the protagonist, is 24 when the novel begins. She moved to New York City from Omaha, Nebraska, several years before the events of the story. She was raised in a devout Catholic family and is one of six children, all of whom have children of their own. Rosemary is a wife and homemaker and aspires to be a mother; she already knows she wants three children, with their births spaced two years apart. Indeed, as she and Guy look for a new apartment, she takes into consideration which rooms could potentially be converted into nurseries.

Rosemary is not close to her family of origin, who disapprove of her marriage to a Protestant, and sees herself as a black sheep. She only speaks to her brother Brian, who also seems to be an outcast. Being raised in a Catholic family clearly had a profound effect on how Rosemary views the world, but at the beginning of the novel, she considers herself agnostic. By the end of the novel, the trauma she experiences at the hands of the cult have awakened some of her old religious devotion, and she prays to God to help her save her baby.