Jessica is a 28-year-old makeup artist in New York City who struggles financially. She takes a spot in a mysterious psychological study on morality to help pay for the medical bills of her younger sister. Jessica feels intense guilt over her sister's condition and carries a heavy burden of secrets. This emotional weight leaves her vulnerable to the study's probing questions.

Dr. Shields is an elegant, wealthy, and highly intelligent psychologist conducting a study on morality and ethics. She presents a clinical, controlled demeanor but harbors deep insecurities regarding her marriage. She uses her professional expertise and financial resources to manipulate her subjects. She views morality through an absolute, clinical lens, yet she ignores her own ethical transgressions.

Thomas is a psychologist and Dr. Shields's estranged husband. He is charismatic but harbors his own secrets, attempting to handle his complicated, tension-filled marriage. Though he occasionally displays a desire to reconcile, he frequently appears on edge and behaves unpredictably.

Becky is Jessica's 22-year-old younger sister who lives in Pennsylvania with their parents. She suffered a traumatic brain injury as a child after falling from a roof. The accident left her permanently disabled and requiring lifelong care. Her condition serves as the primary financial and emotional driver for Jessica's choices.

Noah is a kind, attentive man whom Jessica meets at a lounge. He offers Jessica a sense of normalcy and comfort away from the pressures of her family and the psychological study. However, Jessica's involvement in the experiment constantly threatens to derail their developing connection.

Lizzie is Jessica's best friend, whom she originally met while working backstage at a theater production. She is involved in the theater scene and relies on Jessica for friendship and occasional makeup help. The psychological study quickly drives a wedge of secrecy between them.

April is a 23-year-old former subject in Dr. Shields's psychological study who died by suicide. She acts as a lingering ghost over the current experiment. Her history with the study and her physical similarities to Jessica create an atmosphere of dread. Jessica actively tries to uncover what happened to her during her time as a subject.

Jessica's mother lives in Pennsylvania and acts as the primary caretaker for Becky. She remains optimistic about small milestones but is weighed down by the family's financial burdens and the lifelong responsibility of caring for her disabled daughter.

Jessica's father is a hardworking man facing extreme stress after his company is bought out. This threatens his employment and the family's financial stability. He deeply loves his daughters but struggles under the weight of their current situation.

Ben is Dr. Shields's assistant who manages the logistics of the psychological experiments. He acts as the gatekeeper to the study, processing subjects and handling administrative tasks, though he maintains a nervous, cautious demeanor.

Taylor is a college student who unwittingly provides Jessica with the opportunity to join Dr. Shields's study. She sets the plot into motion by canceling her appointment for the paid survey within earshot of her makeup artist.

Amy is a college student and classmate of Taylor's who previously participated in the first phase of Dr. Shields's study. She provides Jessica with early background information about the professor's academic standing and leave of absence.

Gene is Jessica's former boss from her days working in theater productions. He sexually assaulted her, an event that continues to influence her reactions to certain men and her ongoing struggle to confront uncomfortable truths from her past.

Dr. Shields's mother is a former beauty queen who shares a favorite restaurant with her family. She carries the ongoing grief of losing her teenage daughter in a tragic accident years ago.

Dr. Shields's father is described as a powerful man with connections in the business world. Dr. Shields uses his influence as a dangling promise to manipulate Jessica into staying in the study.

Dr. Shields's unnamed sister died in a drunk driving accident when she was sixteen. Dr. Shields views her sister as selfish and immoral, holding onto the belief that her sister's poor choices directly led to her tragic death.

April's mother is a grieving woman desperately seeking answers about her daughter's sudden suicide. Frustrated by Dr. Shields's refusal to share medical files citing confidentiality, she hires a private investigator to uncover the truth.

Tiffani is a frantic, thin woman who appears on Thomas's call list. She lives with an aggressive, paranoid boyfriend and tries to hide a bruise on her forehead during her makeup session.