In the short story The Witness for the Prosecution, Janet Mackenzie is the maid of Emily French.
Janet is a elder maid that lived with French for 20 years so she was too devoted to her. Her statement about Vole shows that she dislikes him. She said that he convinced French to change her will for his benefit, also he promised her that he would marry her even thought Vole claimed that she knew about his wife Romaine. However the night of the murder, Janet was at church but she returned home at 9:30pm to get a sleeve pattern for a friend, then found the body, Vole remarks that he left 10 minutes before which contradicts Janet's statement. She testifies in court but after her age leads her into confusion her statement is throw away.
The character was later reworked for Christie's own stage adaptation.