Mary Debenham, the tormented and unhappy spinster stock character, is actually the most attractive and interesting female on the Orient Express. Poirot describes Mary as "cool and efficient," a formal and somewhat uncaring English lady; however, Mary is also revealed as an extremely passionate woman. Poirot is attracted to Mary's defiance and quick mind, when she enters the dining car for questioning, he describes her, saying, "She wore not hat. Her head was thrown back...The sweep of her hair back from her face, the curve of her nostril suggested...a ship plunging gallantly into a rough sea...she was beautiful."
Mary is an odd combination of a cool, plain governess and femme fatale. When executing his questioning, Poirot is hardest on Mary—he gives her no breaks and no apologies and his behavior is much different with her than with the other passengers because he know just how strong she is and how difficult she will be to break. Mary was the principle planner of the murder, if anyone she is Poirot's closest match. Poirot does not defeat her until his second round of questioning. This second round is like Mary's last stand, it is the same described above where she is "beautiful." The pressure seems to finally crack her iron demeanor and it is up to Colonel Arbuthnot to defend her.
The fact that Mary is in love with Colonel Arbuthnot does make her elevate her above the typical spinster character. The spinster often isn't a very lovable character, but Mary clearly can be loved and wants to be loved. The presence of a male partner makes the female seem gentler, more digestible. Because Mary breaks into tears and Colonel Arbuthnot must rescue her, the reader feels more sympathy for Mary—she is not too independent or masculine.