In the novel Postern of Fate, Alexander Richard Parkinson was a boy who lived in Swallow's Nest in Hollowquay some time before the First World War. The Beresfords later moved into the house by which time it had been renamed "The Laurels". While unpacking and rearraning, Tuppence Beresford came across a coded message left by Alexander in a children's book which alleged that "Mary Jordan did not die naturally. It was one of us. I think I know which." Finding out who Mary Jordan and Alexander Parkinson was forms the main plot of the book.
Hannibal helps Tommy locate Alexander's grave in the local churchyard which shows that he died young at the age of fourteen. Subsequently, Tommy and Tuppence also discover that Alexander's godmother is Mrs Griffin who is able to provide some background information on the Parkinson family and further contacts.
So far as is known or revealed in the book, Alexander died of natural causes. Beatrice, who works as a cleaner for the Beresfords, tells Tuppence (all from fourth-fifth hand hearsay) that he died from some disease involving the blood where the treatment was "they take the blood away from you, and give you some good blood again." She thought the disease Christian name. Tuppence at first guessed Hodgkin's but Beatrice thought it was a Christian first name. Later, she seemed to agree when Tuppence guessed leukemia. On the other hand, Mrs Griffin later tells Tuppence it wasn't really an illness but something to do with food he had taken while on a picnic. Mrs Griffin heard this from Mrs Henderson who "remembers a lot about the Parkinsons". What happened to Alexander is never confirmed because apart from the coded message, Alexander doesn't actually play much of a role in the rest of the plot.
Alexander is the grandson of a great-aunt of Mr Robinson. A sketch of Alexander, done by Mary Jordan who was his governess, hangs in Robinson's dining room.