Hercule Poirot is a short, balding, and rather physically nondistinctive man who excels at digging beneath the surface to solve seemingly impossible crimes. His one distinct physical attribute is his luxuriant moustache, about which he is rather vain. He is deeply intuitive and invariably smarter than anyone around him, and though he can be snobby at times, he is also a likable fellow. The Belgian detective is traveling on the Orient Express when a fellow traveler is murdered; as he gathers evidence by examining the crime scene, interviewing the passengers and train staff, and using his reasoning skills, his rigor and exactitude never falter. Poirot has a deep-rooted sense of morality and cannot let evil go unpunished; he doesn't always obey written laws and instead follows his own sense of what is right and just.
Mary Debenham is a young English governess who appears to fit the unhappy, uptight spinster stereotype. But in truth she is passionate and forceful. Although she initially hides her former role as governess to Daisy Armstrong, she piques Poirot's suspicion when he overhears her make an impassioned plea to Colonel Arbuthnot, with whom she is in love. Because she is less than forthcoming and refuses to explain the overheard conversation, she becomes a top suspect in the murder of Samuel Ratchett, and indeed she plays a central role in his death.
M. Bouc knows Hercule Poirot from their days working on the Belgian police force. Bouc asks Poirot to investigate the train murder; he is extremely concerned about how the murder will affect the train's reputation and wants to minimize negative publicity. Compared to the brilliant and incisive Poirot, Bouc is rather dense and easily confused, to amusing effect; he gives Poirot ample opportunities to explain his thinking—both to Bouc and the reader. Unlike Poirot, Bouc is given to sweeping generalizations about others based on their nationality or other surface traits.
After Samuel Edward Ratchett is acquitted of murder, he travels abroad on the Orient Express under an assumed name and asks Poirot to assist him because he has received threatening letters and fears being attacked. On the second night of his journey he is stabbed to death by 12 passengers who act as a self-appointed jury. Ratchett appears to have no conscience, and Poirot describes him as a "wild animal."
Mrs. Caroline Hubbard plays a pivotal role in planning and executing Ratchett's murder and very nearly escapes suspicion thanks to her acting skills. When Poirot presents his two solutions to Ratchett's murder, Mrs. Hubbard reveals her true identity and describes how Ratchett's acquittal affected her and other people associated with the Armstrong murder. She explains how they decided to take justice into their own hands and punish Cassetti.