Halsey

In the short story The Adventure of the Dartmoor Bungalow (later, chapters 3 and 4 of The Big Four), Halsey is mentioned by John Ingles. He would appear to be a mutual friend of Ingles and Poirot. He had, it seemed, provided a letter of introduction for Poirot to Ingles when Poirot needed to consult him for background information on Li Chang Yen.