Claire is the introverted and anxious younger sister of Alison. Following the tragedy of her sister's disappearance, she reinvents herself using her middle name, Emily. She moves to New York City after college and works as an editorial assistant. Living in a basement studio in Flatbush, she struggles with obsessive tendencies and spends her evenings surveilling a taxi driver she recognizes from her childhood.

Alison is a highly self-aware college freshman vacationing with her affluent family at the Indigo Bay resort. She is acutely conscious of class and privilege, often feeling uncomfortable with the dynamics of the resort staff serving her family. Her sudden disappearance on the island serves as the catalyst for the novel's events.

Clive is a former resident of Saint X who now works as a taxi driver in New York City. Growing up, he was a shy, heavy-set boy who stuttered, finding his confidence largely through his friendship with Edwin. Now living a lonely life in Flatbush, he is haunted by his past as a suspect in Alison's disappearance and wishes to return to his island home.

Edwin is a charismatic local who works on the beach at Indigo Bay. Unlike his best friend Clive, Edwin dreams of leaving Saint X for New York City and frequently expresses resentment toward the affluent American tourists, even as he caters to them. He is a commanding presence who steers the direction of Clive's life.

Sara is an articulate young woman who grows up on Saint X under the care of a difficult mother. She presents herself with a dignity that hints at a higher social class. Her youthful decisions intertwine her life permanently with Clive's, shaping his responsibilities before he leaves the island.

Officer Roy Cannadine is a longtime police officer on Saint X. He maintains a paternalistic relationship with the local youth, giving them second chances rather than harsh punishments when they get into minor trouble.

Referred to largely by his home state or as the blond boy, he is a Yale student vacationing at the resort at the same time as the Thomas family. He represents the typical affluent guest and becomes a brief romantic interest for Alison.

Drew is Alison's high school boyfriend back in the United States. He represents her life before college and before the vacation to Saint X, carrying his own grief and confusion following her sudden disappearance.

Agatha is Sara's mother, who moved to Saint X when Sara was an infant. She attempts to project an image of respectability by fabricating stories about her past, though the local community eventually sees through her facade.

Desmond is a childhood companion of Clive and Edwin who grows up to work in the island's tourism industry. He eventually leads tours related to the island's darker history, including the locations tied to Alison's disappearance.

Richard is Alison's former high school English teacher. He provides one of the many conflicting perspectives on Alison's character, viewing her as highly intelligent but alarmingly precocious and boundary-pushing.

Nika is Alison's freshman-year roommate at Princeton University. She represents Alison's brief foray into collegiate life and provides Claire with memories of Alison that clash with the family's perceptions.

Harold is a security guard working at the Indigo Bay resort during the Thomas family's vacation. He is one of the few people to observe Alison quietly on the night she disappeared.

Jackie is Claire's college friend who transitions into city life alongside her. She represents the normal, socially adjusted path that Claire increasingly abandons as her fixation deepens.

Sachin is Clive's roommate in New York City. He shares the cramped living conditions common among immigrants trying to make ends meet in the city. Their living situation is highly volatile.

Also known as Bery, she is a peer of Clive and Edwin from their youth. She grows up to become an artist who installs large sculptures around the city, drawing inspiration from Caribbean folklore.

Selena is an actress cast to portray Alison in the true-crime television special Dying for Fun. She endures an exploitative and abusive production environment that mirrors the media's broader consumption of tragedy.

Caroline is Claire and Alison's aunt. She steps in to provide stability and distraction for Claire during the immediate aftermath of the tragedy, taking the young girl out of the country while her parents process their grief.

Bryan is the son of Clive and Sara, born while they are still teenagers on Saint X. He represents Clive's early transition into adulthood and the responsibilities he struggles to meet.