Major Ellerby

In the novel The Murder of Roger Ackroyd, Major Ellerby was the former employer of John Parker before he took up his post as the butler of Roger Ackroyd. Ellerby was addicted to drugs. Parker accompanied Ellerby on his travels. In Bermuda there had been some trouble and a man had been killed partly due to the fault of Ellerby. The matter had been hushed up but Parker blackmailed Ellerby about it until the day he died.

Poirot used his knowledge of this to break Parker and force him to speak truthfully about the events on the night Roger Ackroyd was killed.