In the novel The A.B.C. Murders, Mary Drower was the niece of Alice Ascher. She works in a rich household and Alice was her only relative. She is a Legion member. She is described as a "pretty dark-haired girl".
When Mary was eleven years old, her mother died, and she went to her aunt, Alice. When she was sixteen, she started in service, but she usually visited her aunt on her days out. Mary took a job as a parlourmaid in Overton because she thought it would be a comfort to her aunt to have her nearby. However, after the death of her aunt she feels that there is no reason she should stay, and that she would like to go to London.
Mary later writes to Poirot, telling him that she had written to Megan Barnard, to suggest that they might be more able to find out who the murderer was if they could work together. Megan had also been thinking of the same thing, and suggested that Mary write to Poirot.
After the formation of the Legion, Poirot asks Mary to go to Andover and talk to the children, as they may have noticed who went in and out of Alice's shop on the day of the murder.
Mary later goes with Poirot, Hastings, and the rest of the Legion, to Doncaster, where the fourth murder is to take place. The Legion is to split up to patrol as many streets as possible in the morning, and later to station themselves at various likely points on the race-course. Poirot asks Hastings to escort Mary, as her name begins with a D.