In the novel 4.50 from Paddington, Miss Ellis is Harold Crackenthorpe's secretary at his office in London. She is described as a "streamlined young woman in a well-cut black suit".
When Inspector Craddock questions Harold about his movements on the day of the murder, Miss Ellis provides some information from an office memorandum calendar. She tells Harold that he was in the office at about 3 o'clock and had dictated some letters, and then he went to Sotheby's sales rooms. She says that he did not return to the office again, but she did have a note to remind him that he was attending the Catering Club dinner that evening.