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.
Iris is described as being less attractive than her sister, but she has a personality that Rosemary would have never had. She has a pale face, black hair, and grey eyes.
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.
Iris remembered being a small child, and eating bread and milk, while Rosemary did lessons at a table. There had also been a trip to the seaside one summer, when Iris had envied Rosemary, because she was older, and could swim.
Iris later went to school, and Rosemary went to Paris to be 'finished'. They saw little of each other, even when Iris came home. Iris spent her time in the schoolroom with Mademoiselle, went for walks in the park, ate supper at nine o'clock, and went to bed at ten.
When Iris was seventeen years old, her mother died, and she went to live with Rosemary and her husband, George Barton. They lived in a house in Elvaston Square. During this time, Iris attended French and German lessons three times a week, and also took domestic science classes. She found life dull sometimes, as there were times when she had nothing to do and no one to talk to. She saw little of Rosemary, who was out a lot of the time.
One week before Rosemary's death, Iris entered Rosemary's sitting-room, and found Rosemary sitting at the writing table and weeping. Iris later found that Rosemary had been writing a letter to her. In this letter, Rosemary wrote that if she died, Iris would inherit her money.
Iris had known since she was a child that Rosemary had inherited money from her godfather, Paul Bennett. However, this was the first time she had heard that the money was to go to her upon Rosemary's death. She thought that this made it less unfair, and realised that deep down, she had been feeling that it was unfair for Rosemary to inherit all the money.
After Rosemary's death, Iris continued living at Elvaston Square. George had suggested that Iris' aunt, Lucilla Drake, should live with them, and chaperon Iris in society. Iris had been thankful not to have to make new plans.
About six months after Rosemary's death, Iris found an unfinished letter written by Rosemary to 'Leopard'. Iris kept the letter, thinking that one day, it might be important to show why Rosemary had committed suicide.
Some months after Rosemary's death, Iris started her "social young girl's life" under the guidance of Lucilla. She attended luncheons, teas, and dances, without enjoying them much. At one of the dances, she met Anthony Browne, who had been a friend of Rosemary's.
After that, Iris and Anthony met at least once a week, at the park, and at dances and dinners. However, Anthony never came to the house at Elvaston Square, and Iris eventually noticed this, and began to wonder why.
One night, George asked Iris to come to the study. He showed her two anonymous letters which he had received. These letters said that Rosemary had not committed suicide, but had been killed. He asked her to remember what had happened on the night of Rosemary's death, and to help him find out if Rosemary had indeed been killed.
Anthony later asks Iris if she trusts him. When she answers that she does, he asks her to marry him without telling anyone about it. Iris says that she cannot do that, as George would be terribly hurt, Aunt Lucilla would never forgive her, and she herself is not of age, being only eighteen.
Spoilers ahead[]
Iris Marle attended two dinners during the events of the novel at the Luxembourg restaurant. Rosemary died during the first one, and George died during the second dinner. After George's death, a paper packet containing traces of cyanide is found under the table. It is revealed that Iris found the paper packet in her bag, thought that someone had planted it there to make it look like she killed George, and dropped it under the table.
Iris survives two other 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 is revealed to be an intelligence agent. Iris tells him that she does not like her money very much, but she will keep a little, so that if she ever wanted to, she could leave him.