The Englishwoman Mary Debenham is the first passenger that we meet in the novel, and of all the passengers on board, Poirot gives Mary the hardest time. Mary was a governess in the Armstrong household, and she and Colonel Armstrong are romantically involved. She first attracts Poirot's attention with her suspicious comments to the Colonel in aboard the Taurus Express in Part 1, Chapter 1.
Poirot characterizes Mary as cool, calm, and collected, though she gets flustered when Poirot pushes her buttons. (He does this frequently in the novel.) Poirot sees in Miss Debenham the cool and rational "Anglo-Saxon brain" that he believes could have plotted such an intricate murder (2.10.54).