Ruth Kettering

In The Mystery of the Blue Train, The Honourable Mrs Ruth Kettering (née Van Aldin) was the daughter of Rufus Van Aldin. She lived with her husband Derek on 160 Curzon Street. She was twenty-eight and had auburn hair.

Characters with a connection to Ruth
Rufus Van Aldin - Ruth’s father, who gave Ruth a precious ruby. He didn’t like the fact that Ruth was married to Derek.

Derek Kettering - Ruth’s husband, who married the latter only for inheritance.

Armand, count de la Roche- Ruth’s lover, an amateur burglar

Mirelle - Derek’s lover, a dancer at Parthenon, who wanted Derek to divorce from Ruth. She had the opportunity to kill Ruth

Ada Mason - Ruth Kettering’s personal maid

Role in the novel (several spoilers ahead)
Ruth Kettering was strangled and disfigured onboard Le Train Bleu, and her ruby "Heart of Fire" was stolen. Derek was arrested for her murder, and then released by Poirot. The murderer of the case was actually Ada, who had killed Ruth with her accomplice.