Cassie is a seventeen-year-old girl working as a waitress in a Colorado diner. She possesses a natural talent for profiling people by reading their behavior, personality, and environment, skills she learned from her mother. Haunted by the unsolved, bloody disappearance of her mother five years ago, Cassie feels disconnected from her paternal family and isolated from normal teenage life. She accepts an offer to join an elite FBI program for gifted teenagers, hoping to use her talents to solve cold cases.

Dean is a seventeen-year-old profiler in the Naturals program who possesses innate skills for studying human psychology. Aloof and standoffish, he keeps others at a distance to protect them, harboring deep-seated fears about his own potential for violence. When profiling crime scenes, he dives directly into the mindset of suspects, referring to the perpetrator as "I" rather than "you."

Michael is a confident, sarcastic emotion-reader in the Naturals program. He comes from a wealthy family and was recruited into the FBI program following his father's legal troubles. He hides his true feelings behind a flirtatious exterior and a constantly shifting wardrobe. This habit makes it difficult for Cassie to profile him accurately.

Special Agent Lacey Locke is an FBI profiler in the behavioral science unit. She serves as the primary trainer for the profilers in the Naturals program, testing their abilities with solved cases, crime scene simulations, and victimology exercises. She maintains a strict demeanor, keeping the teenagers away from her active cases while closely monitoring their progress.

Lia is a human lie detector and an expert liar in the Naturals program. She uses her skills to cause tension and obscure her own traumatic past, having once gone by the name Sadie. She frequently changes her clothing to hide her vulnerabilities and acts openly hostile toward newcomers, though she is intensely loyal to her established household.

Sloane is a Natural who specializes in analyzing statistics, numbers, and patterns. She struggles with social cues and understands language quite literally, often missing sarcasm entirely. Despite her robotic approach to data, she possesses a deep, childlike empathy for the victims of the crimes the group studies.

Special Agent Tanner Briggs is a young, ambitious FBI agent eager to prove himself. He heads the Naturals program, recruiting teenagers with innate psychological abilities to help solve cold cases. He is highly knowledgeable about his recruits' backgrounds and insists on strict rules regarding their involvement in active investigations.

Lorelai is Cassie's mother, a former con artist who used careful observation to pose as a psychic. She taught Cassie how to read a person's behavior, personality, and environment from a young age. She disappeared five years prior to the events of the story, leaving behind a bloody dressing room but no body.

Daniel Redding is an imprisoned serial killer who targeted and tortured twelve women before his apprehension five years ago. His prison interview transcripts serve as a training tool for the Naturals, providing grim insight into the mindset of a violent offender.

Judd is a retired Marine who serves as the live-in caretaker for the Naturals' residence. He provides a stern but reliable presence for the largely unsupervised teenagers, enforcing basic rules while allowing them their space.

Genevieve is the daughter of a United States senator. She becomes the target of an active abduction case, prompting the FBI and the Naturals to race against time to locate her using clues left at various crime scenes.