Mrs Lestrange is seen as a mysterious figure by some residents of St Mary Mead, as no one knows much about her, and she hardly goes out of her house. Reverend Clement describes her as "a woman of the world", and wonders why such a woman would come to stay in a country village.
She is very tall, and her hair is gold, with a tinge of red. Her eyebrows and eyelashes are dark. Her face is described as having something "Sphinxlike" about it, and her eyes are "almost golden in shade". She has the "ease and manner of a well-bred woman", but there is also something incongruous about her.
Mrs Lestrange visits Colonel Protheroe one evening. Parts of the conversation are overheard by Gladys, the kitchen maid. It is not a peaceful conversation, with Mrs Lestrange wanting him to do something, and Colonel Protheroe refusing. When Colonel Protheroe is found dead the next day, Mrs Lestrange comes under suspicion. However, she refuses to tell Inspector Slack what her conversation with Colonel Protheroe was about, only saying that it could not possibly have any bearing on the crime.
Mrs Lestrange is subpoenaed to give evidence at the inquest, but Dr Haydock provides a medical certificate saying she is too ill to attend.
Dr Haydock later tells Reverend Clement in confidence that Mrs Lestrange is a dying woman, and that he gives her a month to live, at the longest.
At the end of the novel, Lettice Protheroe tells Reverend Clement that Mrs Lestrange is her mother, and that Dr Haydock was an old friend of hers, who had been keen on her once. Mrs Lestrange had gone to see Colonel Protheroe to tell him that she was dying, and wanted to see Lettice. Colonel Protheroe refused, saying she had forfeited all claim on Lettice. So she wrote a note to Lettice, and Lettice left the tennis party at Much Benham to meet her.
Lettice also states that she is going abroad with her mother, and that she will be with her, until the end.
In the ITV 2004 adaptation of the novel (Episode 2, Season 1 of Agatha Christie's Marple), the character's name is changed to "Sylvia Lester". She is played by Jane Asher and is portrayed slightly differently from the novel. Here she appears as an alcoholic bon vivant and is not suffering from a terminal illness. Lettice does come to live with her at the end of the show and wants to "reform" her.