Cadence is the eighteen-year-old heiress to the Sinclair family fortune, living in Burlington, Vermont, with her mother. She suffers from severe, chronic migraines and relies on strong pain relievers following a mysterious incident during her fifteenth summer on Beechwood Island. Unlike her relatives, she struggles to maintain the family's facade of perfection and frequently questions their materialism. She returns to the island hoping to piece together her missing memories of that fateful summer.

Gat is a highly intellectual and intensely driven boy of South Asian descent who spends his summers on the Sinclair private island. He possesses a strong sense of social justice, shaped by his observations of poverty abroad and his own experiences in New York. He openly criticizes the Sinclair family's extreme wealth and feels the sting of Harris's quiet racism. His deep connection with Cadence alienates him further from the older generation.

Mirren is Cadence's cousin and a core member of their tight-knit group known as the Liars. She exhibits a kinder, more traditional personality than the others, often trying to smooth over tensions. Despite her gentler nature, she fully participates in their rebellion against the adults, retreating to Cuddledown house to escape the family's endless arguments over property.

Johnny is Cadence's cousin and the final member of the Liars. He brings a lighter, less serious energy to their friend group, often enjoying building with Legos and running around the island. This offers a contrast to the heavy emotional atmosphere of the adult Sinclairs. He is deeply loyal to Gat, refusing to exclude him despite the older generation's clear prejudices.

Harris is the wealthy and domineering patriarch of the Sinclair family. He uses his vast fortune and property, including the houses on Beechwood Island, to manipulate his daughters into begging for his favor. He is intensely proud of his heritage, refuses to tolerate weakness, and exhibits clear prejudice against anyone outside his specific social class and race.

Penny is Cadence's mother and a prime example of the Sinclair family ethic. She strictly enforces the rule that no one should display negative emotions, pushing Cadence to hide her pain behind a perfect exterior. She relies financially on her father and becomes deeply entangled in the bitter arguments over who will inherit the prime real estate on the island.

Tipper is the family matriarch who collects expensive ivory items and helps maintain the flawless Sinclair image. Her sudden death fractures the family's stability, removing the buffer between Harris and his daughters and accelerating their greedy infighting.

Bess is Cadence's aunt, characterized by her aggressive pursuit of the family inheritance. She openly stakes a claim on the valuable Boston house while her father is still alive and complains about her assigned residence on the island. She is overtly hostile toward Gat's presence, stating plainly that he does not belong with them.

Carrie is Cadence's aunt and Johnny's mother. She introduces Ed and Gat to the Sinclair world but frequently bows to her father's oppressive rules to protect her financial future. She is highly anxious and participates fully in the ongoing arguments over who deserves the best houses and inheritance.

Ed is Carrie's partner and Gat's father. As a man of South Asian descent, he faces quiet but distinct hostility from Harris, who makes it clear he is an unwelcome outsider on their private island.

Taft is one of the younger Sinclair cousins, collectively known as the Littles. He is noticeably anxious about his surroundings, calling Cadence to ask for reassurance when he feels frightened by the wind moving through the island houses.

Will is another of the younger cousins on the island. He frequently suffers from intense nightmares during their summer stays, requiring comfort from the older family members.

Liberty is one of the younger Sinclair cousins. She closely follows the traditional Sinclair mold, focusing her attention on typical teenage interests like clothes and boys rather than the family drama around her.

Bonnie is one of the younger Sinclair cousins. Unlike the rest of her perfectly polished family, she harbors a dark fascination with ghosts and bluntly tells Cadence she looks like a dead vampire.

Raquel is Gat's girlfriend who lives in New York. Her existence causes significant tension when Cadence learns about her, complicating the romantic dynamic on the island.