Edgar is a 50-year-old former construction company owner who survives a catastrophic workplace accident that costs him his right arm and leaves him with a severe traumatic brain injury. Dealing with chronic pain, mobility issues, and memory loss, he moves to a rental house called Big Pink on Duma Key to heal. He takes up drawing and painting as a form of therapy to manage his uncontrollable anger and phantom-limb pain.

Elizabeth is a wealthy, nearly 90-year-old heiress who owns most of Duma Key, including the house Edgar rents. She uses a wheelchair, dresses eccentrically in items like blue Converse sneakers, and lives with advanced Alzheimer's disease. A former child prodigy who painted to communicate following a toddler-age brain injury, she serves as a patron of the arts for those visiting the island.

Jerome Wireman is a 40-something former lawyer from the mainland who acts as the primary caretaker for Elizabeth Eastlake. He carries deep emotional trauma from a tragic past involving his family and suffers from a severe head injury that has cost him vision in one eye. He bonds quickly with Edgar over their shared experiences of loss, pain, and life-altering injuries.

Perse is an ancient, malevolent supernatural entity associated with the sea, salt water, and a phantom vessel called the Persephone. She targets individuals with prodigious artistic talents, especially those with traumatic brain injuries, using their creative output to manifest her dark influence. She views human sorrow as a game and uses manipulation to ensnare her victims.

Nan Melda is a young Black woman who works as the devoted nanny to the Eastlake daughters in the 1920s. She acts almost as a mother to young Elizabeth, providing comfort and practical guidance during the child's recovery from a head injury. She wears a headscarf and three silver bracelets.

Jack is a college student at the Florida State University campus in nearby Sarasota who takes a part-time job assisting Edgar on Duma Key. He is polite, clean-cut, and hardworking. He quickly becomes a reliable presence in Edgar's life, running errands and helping manage the logistics of Edgar's art career.

Ilse is Edgar's 19-year-old younger daughter, currently studying at Brown University. She shares a uniquely close and tender bond with her father, often acting as a peacemaker within the family following his accident. She worries deeply about Edgar's well-being and frequently checks in on his recovery.

Pam is Edgar's ex-wife, who leaves him after his traumatic brain injury causes uncontrollable and dangerous bursts of anger. Despite their separation, she remains connected to him through their daughters and the complicated aftermath of their long marriage.

Melinda is Edgar's older daughter who lives in France with her boyfriend. She is deeply affected by her father's injury and subsequent personality changes, experiencing the ripple effects of his trauma from a distance.

Kamen is Edgar's Minnesota-based psychotherapist who helps him treat his severe anger issues stemming from his brain injury. He suggests the change of scenery that leads Edgar to Florida and uses unconventional methods, like an anger management doll, to help his patient cope.

Tom is the lawyer who manages Edgar and Pam's divorce settlements. He is astonished when Edgar willingly hands over the vast majority of their wealth to Pam. He later develops a personal connection with Pam.

John is Elizabeth's father, an early 20th-century millionaire and amateur skin-diver who built the family's estate on Duma Key. He provides for his large family of daughters but carries the prejudices of his era and dictates the lives of those on his property.

Mary is a prominent local art critic in Florida who takes a strong interest in Edgar's work. She compares his raw, self-taught talent to famous artists and uses her influence to boost his reputation in the mainland art scene.

Candy Brown is a man arrested for a brutal crime on the mainland. His appearance on the news deeply affects the residents of Duma Key, triggering emotional responses from both Edgar and Wireman as they process the existence of real-world monsters.