Poirot is a semi-retired Belgian private detective living in England, famous for his intellectual prowess and fastidiously maintained mustaches. He relies on psychological analysis and the observation of human behavior rather than purely physical clues. When challenged by an anonymous letter writer signing themselves "ABC," he approaches the emerging mystery with a methodical focus on the perpetrator's underlying motives.

Hastings is a former Army officer and Poirot's loyal friend who documents their investigations. He possesses a straightforward, traditional view of crime-solving and frequently grows impatient with Poirot's psychological methods. He is highly observant of physical appearances, particularly when interacting with attractive young women, but often misses the deeper behavioral subtext surrounding a case.

Cust is a shy, awkward military veteran who works as a door-to-door stocking salesman. He suffers from epilepsy and frequent headaches, conditions he attributes to his wartime service. He travels across England according to a strict schedule, making him a person of interest as his itinerary perfectly aligns with the targeted cities mentioned in the anonymous letters sent to Poirot.

Franklin is Sir Carmichael's younger brother, who projects a confident, energetic, and slightly boyish demeanor. He takes an immediate, active role in organizing the families affected by the mystery into a "Special Legion" to aid Poirot. He expresses strong dissatisfaction with the official police investigation and prefers Poirot's methods.

Alice is an elderly former domestic servant who runs a small tobacco and newspaper shop. She financially supports her estranged, alcoholic husband to keep him at a distance. She is the first individual targeted in the sequence of events outlined by the "ABC" letters.

Betty is a young, outgoing waitress working in the seaside resort town of Bexhill-on-Sea. She enjoys socializing and flirting, behavior that frequently causes friction with her fiancé and her older sister. She becomes the second individual caught up in the alphabetical mystery.

Megan is Betty's older sister, characterized by her dark hair, angular features, and highly observant nature. She works as a typist in London and adopts a frank, realistic view of her sister's flaws. She actively collaborates with Poirot, seeing through his conversational traps more easily than the police do.

Donald is a quiet, freckled real estate employee engaged to Betty Barnard. He harbors deep insecurities and a volatile temper regarding Betty's casual flirtations with other men. He struggles with intense guilt and disturbing dreams as the investigation into the Bexhill incident progresses.

Sir Carmichael is a wealthy, retired doctor and prominent collector of Chinese art living in Churston. He spends his days managing his collections and caring for his terminally ill wife. He represents the letter "C" in the alphabetical sequence investigated by Poirot.

Lady Charlotte is Sir Carmichael's wife, who suffers from terminal cancer and remains heavily medicated. Despite her physical decline, she maintains a sharp, critical eye on the social dynamics within her household. She intensely distrusts her husband's secretary and works to protect her family's social standing.

Thora is Sir Carmichael's young, strikingly blonde secretary who assists with his extensive art collection. She is professionally competent but faces suspicion from Lady Clarke regarding her social ambitions. She provides the investigators with their first physical description of a suspicious door-to-door salesman.

Mary is a young domestic servant and the niece of Alice Ascher. She is deeply attached to her aunt, who was her sole surviving relative. She proves eager to assist Poirot, joining the group of family members coordinating with him in London.

Doctor Thompson is a police psychiatrist who consults on the psychological profile of the letter writer. He believes the perpetrator operates with a specific, rigid internal logic and debates with Poirot over the exact nature of the suspect's mental state.

Inspector Japp is a veteran Scotland Yard detective and long-time acquaintance of Poirot. He holds a practical, working-class view of police work and initially dismisses the anonymous letters as a prank, though he maintains a steady working relationship with the Belgian detective.

Crome is a young, ambitious Scotland Yard detective assigned to the serial case. He acts with an air of superiority and frequently patronizes Poirot, placing his faith in standardized police procedure rather than the older detective's behavioral observations.

Franz is Alice Ascher's estranged, alcoholic husband. He frequently demands money from her and has a history of making public threats, making him an immediate subject of police interest when an incident occurs at her shop.

Mrs. Marbury is the owner of the lodging house where Alexander Bonaparte Cust rents a room. She begins to notice discrepancies in her tenant's behavior but feels guilty for suspecting him of any wrongdoing.

Lily is Mrs. Marbury's observant daughter. She notices Cust's erratic behavior and points out inconsistencies in his travel stories to her boyfriend, eventually prompting a report to the police.

Tom is Lily Marbury's boyfriend. He witnesses Cust dropping a train ticket that contradicts his stated travel plans, a detail he later brings directly to Inspector Crome.