In the novel Sad Cypress, Elinor Katherine Carlisle is the niece of Laura Welman. At the time of the events of the novel, her father and mother had already passed away. She is put on trial for the murder of Mary Gerrard.
In the years before Elinor's parents had died, they had lived in India, and she had gone to Hunterbury for holidays. She played in the woods, plucked sweetpeas, gooseberries, raspberries and apples and curled up in secret lairs reading for hours. She had always felt sure of living there permanently someday.
Elinor is described as having a graceful head, whose bones are "sharp and well-defined". Her eyes are a "deep vivid blue", and she has black hair. Her brows are plucked to a "faint thin line". When Poirot meets her, he observes her "sensitive intelligent face", "square, white forehead" and "delicate modelling of the ears and nose". She is reserved, and does not reveal much about her thoughts and feelings.
Elinor is in love with Roderick Welman. When she sees him, she experiences a "slightly giddy feeling, a throb of sudden pleasure", and a "feeling that it was incumbent upon her to be very matter-of-fact and unemotional". She knows that although he loves her, he does not feel about her the way she feels about him.
At the beginning of the novel, Elinor receives an anonymous letter, telling her that someone was "sucking up" to her aunt, and that she should go down to Hunterbury to see what was going on. She and Roderick travel to Hunterbury to see Mrs Welman, and to tell her that they are engaged.
After Mrs Welman has a second stroke, Elinor and Roderick travel to Hunterbury again. She feels that Roderick is playing the part of the devoted fiancé, and there is a kind of constraint between them. She sits in the sickroom with Mrs Welman while Roderick and the nurses eat dinner, as she feels that she cannot sit through a meal with him, behaving as normal.
Elinor and Roderick break off their engagement, and she advises him to go abroad for a time, so that he will know if he is really in love with Mary or if it is a temporary infatuation.