In the novel Dumb Witness, Miss Isabel Tripp is an amateur spiritualist who shares a home with her sister Julia near Little Green House, the house of Emily Arundell. The latter did not care much about the Tripp sisters. She found their clothes ridiculous, their habits of eating vegetarian and uncooked meals absurd, and their manner affected. She described them as "women of no traditions, of no roots". However, she was amused by the fact that the Tripps were friends with her companion Wilhelmina and let the thing be.
She was described as being "less buxom than her sister" and that she might have indeed described as scraggy. She had very fair hair done up into a large quantity of rather messy curls, along with a "girlish manner".
One night, Isabel functioned as a medium between "E. A." (Emily Arundell) and the deceased General Arundell. It was "a beautiful message, so full of love and comfort", but Miss Arundell knew for a fact that those sentimental messages were not typical characteristics of him. The Tripp sisters later answered that the "Dear Ones" change a lot after the death.
Poirot presented himself to the sisters along with Captain Hastings, and Julia introduced them to Isabel as "Mr Parrot and Captain Hawkins". Hastings, as far as his intuition told him, said to himself that the Misses Tripp were "vegetarians, theosophists, British Israelites, Christian Scientists, spiritualists and enthusiastic amateur photographers".
The two sisters gave some vital information on Emily's death. A "luminous haze" gradually surrounded her head, "an aureole of fresh light". It is later revealed that the sisters had witnessed the effects of phosphorus poisoning.
She was portrayed by Pauline Jameson in the 2002 ITV TV adaptation which made part of the series Agatha Christie's Poirot. Here she has a larger role than in the original. After Emily's death, her fox terrier Bob is gifted to Poirot and not Hastings like in the original story. The two later give the dog to Isabel who mentioned that she had the spirit of a dog in the cottage who wouldn't have let her have another pet, and Poirot pretended to talk with the deceased dog's spirit to convince her.
In Témoin muet, the France Televisions adaptation of the novel which formed part of the series Les Petits Meurtres d'Agatha Christie, Isabel Tripp's role is largely taken by the detective's mother Alexina Laurence. Here Alexina did not have a sister, but a husband who had passed away. In another adaptation of Christie's works (in the same TV series) it is revealed that he had committed suicide.