Mallory is a 21-year-old recovering addict living at a Safe Harbor home in Philadelphia. Once a dedicated cross-country runner with a college athletic offer, she maintains a strict running regimen to exercise discipline and control over her body. She accepts a summer position as a nanny in Spring Brook, hoping to rebuild her stability while keeping her past hidden from her new employers.

Teddy is the five-year-old son of Ted and Caroline Maxwell. He possesses a vivid imagination and frequently plays make-believe games in the woods bordering his property. While generally sweet and cooperative, he fixates on an imaginary friend and produces increasingly disturbing charcoal drawings that alarm his new nanny.

Caroline is a doctor at the Veterans Affairs hospital and the mother of Teddy. She presents as an intelligent and attentive parent who fiercely guards her family's privacy. She establishes firm boundaries regarding religion and screen time, projecting an image of strict domestic control and scientific skepticism.

Ted is Caroline’s husband, a businessman who outwardly projects confidence and affluence. He initially treats Mallory with aloof suspicion but gradually warms up to her presence in the house. He aggressively rejects spiritual concepts, insisting on rational explanations for unusual events occurring on the property.

Mitzi is the Maxwells' eccentric next-door neighbor. She works as a spiritualist who reads auras and claims to communicate with the dead. She frequently gossips about her neighbors and takes a keen interest in the history of the guest cottage, offering a direct contrast to the Maxwells' secularism.

Russell is a 68-year-old running coach and a recovering addict who spent five years in prison. He serves as Mallory’s sponsor in her recovery program. He actively checks on her physical training and mental well-being, providing a reliable anchor to her sober life in Philadelphia.

Adrian is a young man who works for his father’s successful landscaping business. He shares an instant chemistry with Mallory and bonds with her over their mutual interests. He approaches the strange events at the cottage with an open mind, using logic and deductive reasoning to help Mallory investigate.

Anya is Teddy's imaginary friend. According to Teddy, she sings to him at night and acts out stories that he then draws on paper. Her presence becomes a point of contention in the Maxwell household as her associated imagery grows increasingly violent.

Annie Barrett is an artist from the post-World War II era who previously lived in the Maxwells' guest cottage. Local lore suggests she vanished or was murdered there decades ago. Her story captivates Mallory, who suspects her spirit might be communicating through Teddy's drawings.

Sofia is Adrian's mother, a fourth-grade teacher who also works part-time at a local library. She is welcoming but observant, taking an active interest in her son's new romantic prospect and assisting Mallory with historical research.

Beth is Mallory’s deceased younger sister. She died in a tragic car accident while Mallory was driving, an event that deeply traumatized Mallory and accelerated her descent into heavy drug use. Her memory acts as a constant source of guilt and motivation for Mallory's recovery.