Characters with a connection to John
Mary Cavendish - John's wife
Lawrence Cavendish - John's younger brother
Mr Cavendish - John's father, died before the events of the novel
Mrs Cavendish - John's mother. Never appears in the novel.
Emily Inglethorp - Mr Cavendish's second wife, John's step mother
Cynthia Murdoch - no connection until Curtain, when it is revealed that John is Cynthia's brother in law, as the latter had married Lawrence
Role in the novel and the character's fate (as said in Curtain)
He had gone to trial for poisoning Emily because the real murderer had faked his handwriting in a receipt.
In Agatha Christie's Poirot, John Cavendish was portrayed by David Rintoul.