Cynthia Murdoch

In the novel The Mysterious Affair at Styles, Cynthia Murdoch is a girl in her twenties with auburn hair, the daughter of a deceased friend of the Cavendish family. She performs war-time work at a nearby hospital's pharmacy. She lives as a guest at Styles Court. Her room at Styles is adjacent to Emily Inglethorp's.

She is good at disguising, which made her a suspect for Emily Inglethorp's death.

Cynthia is a close friend of "Nibs", with whom she works. When Arthur Hastings first sees Cynthia, he jokingly asks her "how many people does she poison", and she replies with "lots of them".

Spoilers ahead
The day of Emily's death, Mary Cavendish had added a sedative to Cynthia's coffee to enter Emily's room. However, Mary didn't want to kill Emily; she was looking for a proof oh her husband John's infidelity with another woman.

When Emily Inglethorp was poisoned with strychnine, Lawrence Cavendish (John's brother) believed that the murderer was Cynthia. In Curtain, it is mentioned that Lawrence and Cynthia got married and went to live abroad, probably in South Africa.

Agatha Christie's Poirot
In the Agatha Christie's Poirot adaptation, the character of Cynthia Murdoch was portrayed by Allie Byrne. Her role is faithful to the one in the original novel, and the episode itself is one of the most accurate adaptations ever made in the series Agatha Christie's Poirot.

Les Petits Meurtres d'Agatha Christie
In La Mysteriéuse affaire de Styles, the France Televisions and Escazal Films adaptation of the novel, there is a similar character called Dorothée Mellul, who was adopted by Èmilie Beauregard.