In Chapter 7 he is referred to as "Jack" by his sister Beryl. His real name was Rodger Baskerville, before he changed his name to Vandelour and then Stapleton. His "sister", of course, was actually his wife, whose name was Beryl Garcia Baskerville.
Rodger Baskerville Jr.
("My inquiries show beyond all question that the family portrait did not lie, and that this fellow was indeed a Baskerville. He was a son of that Rodger Baskerville, the younger brother of Sir Charles, who fled with a sinister reputation to South America, where he was said to have died unmarried. He did, as a matter of fact, marry, and had one child, this fellow, whose real name is the same as his father's.")
jack stapleton but his real name is rodger baskerville