John Cavendish

In The Mysterious Affair at Styles, John Cavendish is one of Emily Inglethorp's stepsons, along with Lawrence. He is the husband of Mary Cavendish.

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.

Arthur Hastings mentions that John dies before the events of Curtain (presumably in the sixties or seventies), and his widowed wife goes to live in Devonshire.

Portrayals
In Agatha Christie's Poirot, John Cavendish was portrayed by David Rintoul.