Iris Marle

In the novel Sparkling Cyanide, Iris Marle is the younger sister of the deceased Rosemary Barton. Iris was six years younger than Rosemary, and heiress to her sister's fortune. If Iris died, the fortune would be inherited by Lucilla Drake, and eventually, Victor Drake, the black sheep of the family. She was described as being less attractive than her sister, but she had a personality that Rosemary would have never had. Iris and Rosemary were the daughters of Viola and Hector Marle. Hector died when Iris was five years old (and, thus, Rosemary was around eleven at the time), and Viola's health was delicate, so the two sisters spent their time with governesses. Viola and the Marle family in general used to give more importance to Rosemary than to Iris, and she herself wasn't too jealous (apparently) or worried. During her teenage years, she attended "Domestic economy", French and German lessons three times a week.

When she was seventeen, Iris Marle went to live in a house in Elvaston Square with her aunt Lucilla and Rosemary's husband George Barton. Iris mentioned how she and her sister weren't on intimate terms, so she couldn't know what led to her older sister's suicide. One week before Rosemary's death, Iris saw Rosemary writing her last will and thought that it was strange that a young woman like Rosemary Barton thought of her death. There she learned that Paul Bennett left all his fortune to Rosemary, and if Rosemary had died without children her fortune would have passed to her younger sister. When Rosemary killed herself, everyone believed she suffered of post-infulenzal depression. It later turns out she was murdered.

Iris also found one unfinished letter written by Rosemary to someone unknown in which she threatened to kill herself. Some months after her sister's death, Iris Marle became acquainted with an Anthony Browne. The two later became good friends and recalled George Barton's attention who was suspicious about Browne having killed George's wife. At that period in fact someone had written anonymous letters saying that Rosemary was killed and did not commit suicide. The only people to know this (apart from who had written the letters) were George and Iris, who was on intimate terms with her brother-in-law.

Spoilers ahead
Iris Marle attended two dinners during the events of the novel at the Luxembourg restaurant. Rosemary died during the first one, and Iris' brother-in-law died during the second dinner. However, the poison with which George Barton was killed was found in Iris Marle's possession because she was actually the intended victim.

Iris survives other two murder attempts. At first she is almost ran over by a car, and then she is almost asphyxiated to death. All three attempts fail. The murderer of the case is revealed to be Ruth Lessing, the secretary of George.

At the end of the novel Iris gets engaged to Anthony, who was revealed to be an intelligence agent.