Lucy Jerrard is a non-canonical character who was created for the adaptation of In a Glass Darkly, the second episode of the Thames Television series The Agatha Christie Hour. She is a guest during the engagement party at Badgeworthy which was organised for Charles Crawley and Sylvia Carslake. Neil Carslake interrupted her in order to introduce his friend Matthew Armitage to his sister Sylvia.
Mrs Carslake said of the guests that they are "all family" so Lucy must have been a relative of some kind.
Lucy Jerrard was played in the film by Sarah-Jane Bickerton. This character does not exist in the original story.