In the short stories Poirot and the Regatta Mystery and the Parker Pyne short story The Regatta Mystery, Evan Llewellyn is a "dark, young man" with "a faintly hungry wolfish look abut him which some women found attractive." He is a member of the yachting party hosted by Isaac Pointz at Dartmouth. He attended the dinner at the Royal George Hotel during which the Pointz's diamond, “the Morning Star” went missing.
His host, Isaac Pointz is suspicious of Evan because after his diamond went missing, when young Eve Leathern plays a trick on Mr. Pointz, pretending to make his diamond disappear, Evan had gone to the window after the diamond had "disappeared", tossing down a coin and a vendor tosses back up a newspaper. Because this act is seen as the only way that the diamond could have left the room, Mr. Pointz sends an investigator to interview Evan. He is upset because he's afraid that the suspicion of being the thief will not only follow him forever, but has put him in bad graces with Janet Rustington, whose first husband was "a rotter."
Evan comes to Parker Pyne to help him solve the disappearance of the diamond in order to clear his name.