The main character is Hercule Poirot. He is a famous detective with remarkable deductive skills.
There is also Ratchett, the victim in the story. His death sets off the mystery. And there are a bunch of passengers on the Orient Express, each with their own secrets and motives, like the Princess Dragomiroff, who is an aristocrat with a haughty demeanor.
Another important character is the conductor. He has a role in the events that unfold on the train. The various passengers, including Colonel Arbuthnot and Mary Debenham, are all intertwined in the complex web of the murder mystery.