Cust started as a red herring of this novel's culprit, appearing in specific chapters not narrated by Hastings, up until he crossed his path Poirot and Hastings in the final chapters.
He was originally speculated as the ABC Killer because of his presence in all of the crime scenes, as well as his full name, but it was revealed that he was an innocent man who was actually manipulated and framed by the real killer.
Alexander Bonaparte Cust is a travelling salesman with a pompous name. He sells stockings and is epileptic. He fought in the war, and received a blow on head, making him prone to blackouts and severe headaches. He is a suspect in the murders of Alice Ascher, Betty Barnard, Carmichael Clarke and George Earlsfield. Even though he had an alibi for the murder of Betty Barnard, the police said the alibi was fake.
Nevertheless, after some delve into the case, it was revealed that Cust was actually innocent in all of the cases above, and was merely a red herring manipulated by the culprit. He was hired by the real A.B.C. Killer under the impression of a company and went to the four crime scenes solely for business.
However, as the news of the ABC Murder began to spread, Cust began to suffer from paranoia and distress because of his illness as well as his fear of being killing people. After the first three murders, Cust arrived at Doncaster under the culprit's manipulation, with the latter killed Earlsfield randomly and put the blade inside Cust's pocket when he was unawared. Thus, Cust was framed and arrested, but Poirot managed to discover the real killer by judging through his letters, before identifying the real murderer and cleared Cust's name.
Cust was eventually released and was cured from his condition. He paid visit to Poirot and Captain Arthur Hastings to give his thanks at the end of the story.