In the Harley Quin short story The World's End, The Duchess of Leith is a fellow traveller whom Mr Satterthwaite encounters at Cannes. Described as "a very authentic Duchess", she is the daughter of a Duke and the wife of another. She is, however, rather shabbily dressed, with black bead trimmings and quantities of diamonds in old fashioned settings which looked like they had been flung at her in a haphazard manner.
Although the Duchess donated generously to charities, she was nonetheless very miserly about herself. After a quarrel with the hotel manager about the price of her room, she decides to leave Cannes and go to Corsica and drags Satterthwaite along. Rather than pay for an air ticket, she decides to take an uncomfortable and long ferry crossing and then checks into a fairly low-end hotel.
At the hotel, the Duchess spots a niece Naomi Carlton Smith. She appears to be moping and the Duchess decides that Naomi needs taking out of herself and organises an excursion to see the sights with Naomi providing the transport. Unfortunately, Naomi's car is too old and unreliable to take passengers on the mountain roads. The Duchess is unwilling spend money on a rental car but fortunately she spots a fellow hotel guest, Mr Tomlinson who has a four seater car. Thus begins their excursion to the mountaintop village of Coti Chiaveeri which Naomi calls "The World's End".
In the short story The Harlequin Tea Set, her name is given as Maria Leith. She died some years earlier. It is possible that Mr Satterthwaite and Hercule Poirot attended a gathering at her home in the novella Dead Man's Mirror. Mr Satterthwaite mentions that he saw a lot of the duchess in the novella in Corsica.