Hercule Poirot gets letters warning him of murders to take place in Andover, Bexhill, Churston and Doncaster, all signed ABC. (makes fun of Sherlock Holmes on p.73)
Alexander Bonaparte Cust, a shabby elderly ex-army man who we are led to believe is the ABC murderer. He even believes it himself, but it turns out he isn't. He has epileptic fits and believes he must have murdered the victims (and suffered memory loss), as he was in the vicinity of all the crimes (sent there by the company that employs him as a salesman to hawk their stockings from door to door).
Inspector Japp of Scotland Yard
Mrs Alice Ascher, the first victim, 60 year old tobacconist in Andover, killed by a blow to the back of her head, found slumped on the floor behind her shop counter.
Franz Ascher, her estranged German husband who is a drunkard
Mary Drower, her niece working as a maid in Overton
Inspector Crome of CID, young and rather full of himself
Dr Thompson, the famous alienist
Elizabeth (Betty) Barnard, the second victim, a waitress found strangled with her own belt, on Bexhill beach.
Megan Barnard, her sister
Donald Fraser, EB's boyfriend who couldn't understand why Betty was so flirtatious
Sir Carmichael Clarke, the third victim, killed by a blow to the head while out for a walk late one evening near his house Combeside in Churston, Devon. A rich retired surgeon with a great collection of Chinese pottery.
Lady Clarke, his wife, dying of cancer.
Franklin Clarke, his only brother, recently returned from life in the East. He was afraid that once his sister-in-law died, his brother would marry Miss Grey and start a family (thus ruining his chances of inheriting his brother's fortune). He met A.B. Cust by chance and hit upon the idea of hiding the motive of his brother's murder by making it appear one of a series of murders committed by a madman with an alphabet fetish.
Miss Thora Grey, CC's beautiful young secretary, dismissed by Lady Clarke after her husband's death
George Earlsfield, the fourth victim, a barber stabbed in a cinema in Doncaster on the day of the St Leger horse race.