Frank is a sixteen-year-old boy living on an isolated Scottish island with his father. He views himself as the guardian of the island, maintaining a complex system of gruesome rituals and animal sacrifices to protect his territory. He consults a prophetic device he built called the Wasp Factory to interpret omens. He operates with a severe lack of empathy, casually reflecting on his past murders of three young family members and harboring deep prejudices against women.

Angus is Frank and Eric's eccentric father. He walks with a pronounced limp and spends much of his time cataloging the exact measurements of household items on scattered notes. He restricts Frank's access to a perpetually locked study in the house and subjects his son to bizarre dinnertime lectures about diseases and flatulence. He projects an image of supreme intelligence but often relies on petty manipulation to maintain control.

Eric is Frank's older half-brother. Once an ambitious medical resident, Eric suffered a severe psychological breakdown after a traumatic incident involving an infested infant at the hospital. His mental deterioration led him to terrorize the local community by burning dogs and tormenting children, resulting in his institutionalization. He frequently calls Frank from unknown locations, acting manic and threatening as he makes his way back to the island.

Mrs. Clamp is a local woman who performs menial work and brings groceries to the Cauldhame home. She has assisted the family since Frank was a toddler and is the only woman who regularly enters the household. She occasionally joins Frank and Angus for their traditional Saturday dinners.

Jamie is a twenty-three-year-old man with dwarfism who lives in the local town. He is Frank's primary friend and regular drinking companion. They often discuss politics and the nature of human behavior at the pub, and Jamie expresses heavy anxiety regarding Eric's recent hospital escape.

Diggs is a local detective who periodically checks in on the Cauldhame family. He visits the island to gather information and investigate disturbances, particularly concerning Eric's escape and the history of violent incidents in the area.

Agnes is Frank and Paul's mother. She left Angus shortly after Frank was born, returning only briefly three years later before leaving permanently. During her final departure, she ran over Angus's leg with her car, causing his permanent limp.

Paul is Frank's younger brother, born on the same day that Frank suffered a traumatic dog attack. Frank murdered Paul when the boy was five years old by tricking him into hitting an old, half-buried explosive on the beach with a stick.

Blyth is Frank and Eric's cousin. After Blyth maliciously burned the brothers' pet rabbits with a homemade flamethrower, Frank harbored a deep grudge. When Blyth lost half a leg in a car accident, Frank slipped a venomous snake into Blyth's prosthetic leg while he napped, resulting in his death.

Esmerelda is Frank's young cousin, the daughter of Harmsworth and Morag Stove. Frank decided to kill her simply to balance out the gender ratio of his victims. He rigged a heavy kite with strong cords that wrapped around her wrists and carried her out to sea.

Mary is Eric's mother and Angus's first wife. She died in the Cauldhame house while giving birth to Eric, an event Frank suspects heavily influences Eric's later psychological deterioration.