In The Mystery of the Blue Train, Ada Beatrice Mason is Ruth Kettering's maid. At the time if the events of the novel, she had been with Ruth for two months.
Mason is described as being a tall, thin woman, dressed in black. She wears grey travelling gloves while travelling on the Blue Train.
Mason boards the Blue Train with Ruth, and occupies a compartment next to Ruth's. Katherine Grey sees her there, sitting very upright on the seat and clutching a scarlet morocco case which belongs to Ruth. Katherine later sees Mason receiving a dinner basket through the window of Ruth's compartment.
When Ruth is found dead, Mason is not on the train. According to the conductor, Pierre Michel, Ruth had been obliged to leave Mason behind in Paris. Rufus Van Aldin later picks Mason up from Paris, and brings her to Nice.
When Mason is interviewed by the police in Nice, she has exchanged her grey gloves for black suède ones. She tells the police that at the Gare de Lyon, Ruth had told her that she had changed her plans. Mason was to get off the train, go to the Ritz, and wait there for instructions which would be wired to her. She also says that Ruth had met a gentleman, whom she had brought into her compartment. However, Mason had only seen his back, and so could only say that he was tall and dark.
Mason is later asked to look at the Comte de la Roche, and to say if he was the gentleman whom Ruth had brought into her compartment. She says that it might have been him, but she is not sure.
Poirot later asks Mason if she thinks that the gentleman on the train could have been Derek Kettering. She tells him that it might have been, but she would not like to say. She also tells him that she thinks the cigarette case which had been found in Ruth's compartment was something which Ruth had bought to give to Derek.
Mason later tells Mr Van Aldin that she has thought it over, and is now almost certain that the man in the train was Derek Kettering.
Role in the novel[]
She is revealed to actually be Kitty Kidd, a renowned male impersonator and actress, and an accomplice of The Marquis, who killed Ruth. Kidd pretended to be Ruth and told Pierre Michel that "Ada is getting off the train in Paris". This made Pierre believe Ruth was still alive.
After Ruth was killed, Kidd arranged the body in the bunk to appear like it was sleeping. She then changed into a man's clothes, left the train, and returned to Paris. Poirot was able to get evidence that she had arrived on the morning after the murder, and not on the evening of the day before.