Richard Knighton

In the novel The Mystery of the Blue Train, Major Richard Knighton is Rufus Van Aldin’s secretary. He is described as "quick, intelligent, and resourceful, and he had a disctinct charm of manner". He had been hired by Rufus two months prior to the opening of the story, when they casually met in a Swiss resort.

He speaks French fluently.

Role in the novel (spoilers ahead; do not read if you haven't read the novel yet)
He later turns out to be The Marquis, the murderer of the case. He is in league with Ada Mason. They killed Rufus' daughter to steal the Heart of Fire, a ruby.

A clue that made Poirot think Knighton was the murderer was that there was an ash tray in the victim's compartment; it had the letter "K" on it. Ada declared it was a gift made from Ruth Kettering to her husband. However, this was a lie since the relationship between the couple wasn't good; so the ash tray couldn't have been a gift.

Near the end of the First World War, Knighton was hospitalized in Rosalie Tamplin's villa, which was temporarily set up as a hospital for wounded military personnel. During that period of time, a series of small thefts were happening in Rosalie's house. Those were actually Knighton's first heists. The latter was later exposed by Poirot.

Towards the ending of the novel, he falls in love with Katherine Grey.

Portrayals
In the 2005 ITV Agatha Christie's Poirot episode adapted from The Mystery of the Blue Train, Richard Knighton was portrayed by actor Nicholas Farrell. In this version, Ada Mason tries to kill Katherine because Knighton had fallen in love with her.