Rather than being a character in the story, the narrator is an omniscient third-person voice. The narrator is fairly unobtrusive and sticks to giving us the facts. The narrator also gives us multiple points of view and switches between, for example, the point of view of Poirot, and the point of view of other characters, such as Mary Debenham.
How would this story be different if it were told by Poirot? What if it was told by M. Bouc, Poirot's sidekick, like how Watson narrates the Sherlock Holmes stories?