In the novel Five Little Pigs, Diana Crale is mentioned. She is the daughter of Richard Crale, and the sister of Amyas Crale.
Diana does not appear in person, but is mentioned by Caleb Jonathan, when he is giving Poirot some bakground information on the Crale family. According to Mr Jonathan, Diana was friends with Caroline Spalding, who would later marry Amyas.
Mr Jonathan also mentions that Diana married a man who was not a gentleman, and was a temporary officer during the war. This was a disappointment to her parents.