In the novel Evil Under the Sun, Kenneth Marshall was the husband of Arlena Marshall. He is the father of Linda Marshall. He is about 38 years old. In the register of the Jolly Roger Hotel, his address is given as 73 Upcott Mansions, London, SW7.
Captain Marshall is described as being "fair-haired and suntanned". He has a "quiet pleasant face". He has "straight features", "steady bllue eyes" and a "firm mouth". His voice is low and pleasant. He smokes a pipe.
Rosamund Darnley, a childhood friend of his, claimed that Ken was a fool when it came to love. His first wife had been put on trial, accused of poisoning her then husband with arsenic. After she had been acquitted, Kenneth married her. She died at childbirth a year later, when Linda was born.
Captain Marshall is a quiet man, and does not display his emotions, so it is difficult to know what he feels or thinks.
According to Mr Blatt, Captain Marshall had nearly been arrested for assault once. The other man had "put up a pretty dirty deal", and Captain Marshall had trusted him. Captain Marshall attacked and nearly killed the other man. The other man did not prosecute as he was too afraid of what might happen after that.
When Rosamund asks Captain Marshall why he does not divorce his wife, and points out that people are doing it all the time, he says that he hates that attitude to life. He expresses the opinion that there has to be such a thing as good faith, and that if you marry a woman and engage yourself to look after her, then it is up to you to do it.
On the morning of the murder, Captain Marshall has breakfast at nine o'clock, and looks in on his wife's room after that, to find that she is out. He bathes, and then returns to the hotel. He types letters in his room, and later plays tennis with Rosamund, Mr Gardener and Christine Redfern.
Captain Marshall tells Colonel Weston, Poirot and Inspector Colgate that Arlena was beautiful and that most men admired her, but that many women were envious of her. He also says that he never thought about the affair between his wife and Patrick Redfern.
The police find that the letters which Captain Marshall typed on the morning of the murder could not have been typed in under an hour, and could not have been prepared beforehand, giving him an alibi for the time of the murder. Rosamund also tells Colonel Weston, Poirot and Inspector Colgate that she returned to the hotel and saw Captain Marshall typing in his room at twenty minutes past eleven, although he did not see her.
Captain Marshall later tells them that he did see Rosamund, in the mirror that hung on the wall in his room. However, he did not interrupt his typing to speak to her, as he wanted to finish his letters.
It is later revealed that both Rosamund and Captain Marshall had lied. She had told a lie about seeing him typing in his room as she felt that his alibi was not strong enough, and he had lied about seeing her because he thought that she needed her story corroborated.
At the end of the novel, Captain Marshall asks Rosamund to give up her dress-making business, and live in the country with him.