Charles is a young officer in the English diplomatic service who falls in love with Sophia Leonides while stationed in Cairo. As the son of the Assistant Commissioner of Scotland Yard, he holds a unique position that bridges the police force and the Leonides family. He temporarily moves into Three Gables to observe the family dynamics, acting as an unofficial investigator into Aristide's death while trying to clear the family name so he can marry Sophia.

Sophia is a 22-year-old Foreign Office worker and Aristide's eldest granddaughter. Following her grandfather's suspicious death, she pauses her engagement to Charles, fearing the scandal will ruin his diplomatic career. She is highly protective of her family's reputation and recognizes the unhealthy, tangled dynamics caused by living under her grandfather's absolute control.

Josephine is Sophia’s 12-year-old sister and Aristide’s granddaughter. Kept away from other children at Aristide’s behest, she has a precocious intellect and an obsession with detective novels. She lacks social graces and openly eavesdrops on the household, writing her observations in a little black notebook while proudly claiming the police are incompetent.

Brenda is Aristide’s 34-year-old widow, having married him when he was 75 and she was 24. A former waitress, she faces severe prejudice from the rest of the family due to her working-class background. Because she administered Aristide's daily insulin shots, she immediately becomes the primary suspect in his poisoning.

Aristide is the 85-year-old patriarch of the Leonides family who amassed a massive fortune through highly profitable catering businesses. He emigrated from Greece to England as a young man and built Three Gables, a sprawling mansion where he houses three generations of his descendants. His controlling love keeps his family financially dependent and closely tied together until his sudden death by poisoning.

Edith is Aristide’s elderly sister-in-law from his first marriage to Marcia. Born into wealth, she harbors a class-based disdain for Aristide, considering him a commoner, though she moved into Three Gables to help raise his children after her sister's death. She is fiercely protective of the Leonides lineage and highly observant of the household's toxic dynamics.

Roger is Aristide’s eldest son and his clear favorite. He runs Associated Catering, a business passed down by his father, but he completely lacks the financial acumen to keep it profitable. He feels immense guilt over failing his father and possesses a quick, sometimes violent temper that places him under suspicion.

Philip is Aristide’s younger son. Sidelined by his father’s obvious favoritism toward Roger, Philip has developed a bitter, emotionally repressed, and withdrawn temperament. He spends much of his time reading in the library, maintaining a cold distance from the rest of the household, including his own children.

Clemency is a practical and level-headed scientist married to Roger. She stands in sharp contrast to her highly emotional husband, providing him with stability. She harbors a quiet desire for a simpler life far away from the smothering luxury and tangled dynamics of the Three Gables estate.

Magda is Philip’s wife, a vain theater actress who frequently uses the stage name Magda West. She treats real-life tragedies, including her father-in-law's murder, as opportunities for melodrama and excitement. Her immense egoism causes her to neglect the emotional needs of her children.

Eustace is the 16-year-old son of Philip and Magda. Handsome and bright, he has become deeply moody and bitter since surviving a bout of polio that left him partially paralyzed. He is openly frustrated by his family's eccentricities and restrictive living arrangements, callously remarking that it was time his grandfather died.

Laurence is the live-in tutor for Eustace and Josephine. He is a high-strung, anxious man whom Sophia describes as cowardly and unthreatening. He falls under intense police suspicion due to persistent rumors of a romantic relationship with Aristide's young widow, Brenda.

Arthur is the Assistant Commissioner of Scotland Yard and Charles’s father. He provides Charles with historical context about the Leonides family and authorizes his son to act as an inside observer at Three Gables. He holds a firm philosophy about the psychology of murderers, believing they inherently lack a moral brake.

Taverner is the police inspector leading the official investigation into Aristide’s death. He holds a pessimistic view of the case initially but systematically interviews the family members to uncover a motive. He works closely with Charles, valuing the young man's access to the family.

Janet is the live-in nanny who cares for the youngest Leonides children, particularly Josephine. She is a loyal domestic worker who has spent years embedded in the volatile household of Three Gables, observing the family's many eccentricities.

Mr. Gaitskill is Aristide’s longtime lawyer. He manages the family's legal affairs and helps the police understand the financial implications of Aristide's death for the surviving family members, uncovering irregularities in the paperwork.