Barry is a robbery detective with the NYPD who lives with the heavy grief of losing his teenage daughter, Meghan, in a hit-and-run accident over a decade ago. Divorced from his wife, Julia, he channels his energy into his police work. His encounter with False Memory Syndrome (FMS) forces him to investigate cases where people possess vivid memories of lives they never actually lived.

Helena is a 38-year-old brilliant, workaholic neuroscientist. Motivated by her mother's battle with Alzheimer's disease, she dedicates her career to building a chair that can record and project human memories. When a mysterious benefactor offers her unlimited funding, she relocates to an isolated ocean oil rig, only to discover her employer has entirely different plans for her invention.

Marcus is a 34-year-old influential inventor, business mogul, and philanthropist who operates out of Fawkes Station, a highly secure oil rig 173 miles off the coast. He possesses vast resources and an intense interest in neuroscience. He pushes his scientific team to develop technology far past its original medical intentions, prioritizing his own ambition over safety.

Julia is Barry's ex-wife. Following the tragic loss of their teenage daughter, their marriage dissolved, though they maintain a polite connection anchored by their mutual trauma. She attempts to move forward with her life, eventually remarrying, but remains permanently scarred by her past.

Meghan is Barry and Julia's daughter, a strong-willed girl whose life was cut short by a hit-and-run just before her sixteenth birthday. Her death is the central trauma for her parents. She survives only in their memories and in their speculations about what kind of adult she would have become.

Jee-woon acts as Marcus Slade's neat and professional second-in-command. He handles the logistics of recruiting scientists for the Fawkes Station oil rig and managing the memory chair project. While he initially presents a friendly, corporate demeanor, he ruthlessly enforces Slade's strict rules.

John is a government employee working for DARPA who becomes involved in managing and applying the memory chair technology. Unlike Slade, he genuinely wants to use the invention to correct atrocities and make the world less violent. He views the technology through an idealistic lens, often debating the ethics of its use with his team.

Gwendoline, known as Gwen, is a blonde, regal-looking woman in her mid-thirties who leads a counterterrorism SWAT unit for the NYPD. She and Barry share a comfortable, pragmatic friendship that survives an awkward, drunken one-night stand they mutually pretend never occurred.

Ann is a forty-year-old woman suffering from False Memory Syndrome (FMS). She experiences vivid, black-and-white memories of an alternate life complete with a husband and son. When the people from her alternate memories fail to recognize her, the emotional whiplash drives her to extreme distress.

Joe is a resident of Montauk who becomes tangled in the False Memory Syndrome phenomenon. Ann Voss Peters remembers him as her husband from an alternate life, though he initially denies knowing her when Barry questions him about the connection.

Franny is Joe Behrman's wife. She previously attempted suicide by planning to jump off the exact same building ledge that Ann Voss Peters later utilizes, adding a complicated layer to the FMS mystery.

Vince is a former corrupt NYPD cop who now works as an enforcer for the secretive Hotel Memory. Recognizable by his shaved head, he acts as a guide and muscle for the hotel's mysterious operations, disarming intruders and managing clients.

Reed is a skinny man resembling a heroin addict who serves as a human test subject for Marcus Slade's memory experiments. He receives a fresh tattoo specifically so the scientific team can map the vivid memory of his physical pain.

Dorothy is Helena's mother, residing in an elder care facility in Colorado. Her ongoing battle with Alzheimer's disease serves as the fundamental, deeply personal motivation for her daughter's obsession with memory mapping.

Dr. Wilson is the medical project manager recruited for the memory chair project. He oversees the dangerous physiological aspects of the memory immersion process, including stopping patients' hearts to artificially induce the release of DMT.

Sergei is a Russian electrical engineer employed on Marcus Slade's isolated oil rig. He assists in building the complex microscopic machinery required for the memory chair and provides companionship to Helena during her isolation.

Rajesh is a software engineer working on the memory project at Fawkes Station. He handles the complex programming required to map and project human memories, drawing Helena's professional respect and personal interest.