Mrs Harris is a non-canonical character, created for the 2022 miniseries Why Didn't They Ask Evans?, an adaptation of the Agatha Christie novel of the same name. She is an undisclosed member of staff of the Marchams.
Mrs Harris does not appear on screen, but is only mentioned in passing by Frankie and her mother, as the person whom Frankie asked for permission to borrow her parents' car and chauffeur. Lady Marcham deems this ill-advised, as Mrs Harris is not a parlourmaid and does not know what goes on in the house, so there is no point in asking her anything.