Theresa Arundell

In the novel Dumb Witness, Theresa Arundell is the niece of Emily Arundell and a member of the Arundell family. She has one brother Charles and one cousin Bella Tanios, who is married to a Greek man. Theresa is engaged to Dr Rex Donaldson, an ambitious young doctor. She is about twenty-eight or twenty-nine, tall and slender, with jet-black hair and a heavily made-up dead pale face. She gave Hastings the impression of an exaggerated drawing in black and white. She drinks quite heavily, not for the love of it, but because her friends do--she claims she could stop at any time.

On the Friday before Easter, she stayed at her aunt's house, Little Green House. Her aunt Emily said to herself that since Theresa was 21, she had no more control over her money. She "belonged to a bright, go-ahead set in London" that occasionally came to the attention of the police. She disapproved Theresa's way of living, and her engagement with Dr Donaldson which was "not suitable" for an Arundell. Charles thought the same--she was crazy about Donaldson but she had expensive tastes and he was only a doctor from a quiet English village.

She had a modernly furnished flat in a block at Chelsea, overlooking the river. Poirot and Hastings visited her there, and she said she did not know where her brother Charles lived at the moment, which turned out to be a lie. Poiot asks her a few more questions about the family, and she answers with "Ca ne vous regarde pas, M. Hercule Poirot", mocking his manner. When the two were about to leave, Theresa asked them to come back and offered to pay quite a large sum to break Emily Arundell's will in favour of her companion Wilhelmina.

Theresa's father Thomas had left her and Charles thirty thousand pounds each, which they spent in a short time. Now, the remains of her fortune were only two hundred and twenty-one pounds, fourteen and seven-pence. The two brothers reciprocally suspected each other of the murder. Theresa's manner, bold and ruthless, showed she knew about her aunt's death being murder--Charles, was not only suspected by her sister but by the victim herself. In addition, Miss Lawson saw someone on the stairs of Little Green House placing a nail to make Emily Arundell trip. She saw only the dressing-gown, which had a brooch with the initials T. A. However, she saw all of that in the mirror and the initials were A. T.: Arabella Tanios.

Near the end of the novel, Hastings reveals that Theresa and Dr Donaldson married.

Portrayals
In the ITV TV adaptation which was part of the series Agatha Christie's Poirot, her part was played by Kate Buffery. Here she does not have a fiancé named Rex Donaldson.