In the novel A Caribbean Mystery, Gregory “Greg” Dyson is a nature lover. He especially likes butterflies and birds, and his work involves writing about butterflies. Lucky is his second wife. His first wife died, and he was left a rich man, having inherited a lot of money from her.
Greg is described as being a big man with an "upstanding bush of thick grey hair". He is boisterous and happy-looking.
Miss Marple finds it difficult to match Greg with a village parallel because he is American. However, she thinks of Sir George Trollope, who was full of jokes at Civil Defence meetings, and Mr Murdoch, who had rather a bad reputation, but was said to encourage the rumours.
Greg makes passes at various women, including Molly Kendal and Evelyn Hillingdon. According to Evelyn, he makes passes at everyone, and it is "just part of the Greg he-man act". Señora de Caspearo also says that Greg makes passes all the time.
Greg has high blood pressure, for which he takes Serenite three times a day. He has a larage stock with him, about six bottles. He keeps the one in current use in the open, and the rest locked in a suitcase.
According to Miss Prescott, there had been some scandal when Greg's first wife, Gail, was still alive. Lucky was a relation of Gail's, and had come to the caribbean to join them. She had done some work with Greg, and taken care of Gail, and people talked because Greg and Lucky got on so well together. When Gail died suddenly, Greg married Lucky very soon afterward, which also caused people to talk. Miss Prescott thought that it was only a month after Gail's death that they were married, and she was of the opinion that it seemed "unfeeling".
Greg is later approached by Victoria Johnson, who returns him a bottle of Serenite tablets which he had thought he mislaid. She says that she found them where they had been put, in Major Palgrave's room.
After Lucky's death, Greg flies to one of the other islands, and it is rumoured that he is devoting himself to an Argentinian widow.
In the 1983 Warner Bros adaptation, Greg Dyson is portrayed by Stephen Macht. Here he takes serenite for high blood pressure, just as in the original novel.
In the BBC 1989 adaptation, Greg Dyson is portrayed by Robert Swann.
In the 2013 ITV adaptation, Greg Dyson is portrayed by Charles Mesure. Here he is a wildlife photographer who is working together with Edward Hillingdon who is an ornithologist. Greg does not have a previous wife in this adaptation. Instead, Lucky Dyson had a previous husband, one Parker De Wit III. After he died (under suspicious circumstances), Greg married Lucky. By the time of the events at St. Honoré, Greg already knew that Lucky was no longer in love with him. However, Lucky reminded him that they cannot get divorced because they knew too much about each other and could not trust each other once apart. Greg knew about Lucky poisoning her first husband. And Lucky reminded him that as the second husband, Greg would be considered a suspect or accomplice. Unlike some other portrayals, Greg is here portrayed as somewhat of a lady's man. He claims, for example, that he used to romance Molly Kendal years ago when she was unmarried and at the hotel at that time run by her parents.