Her Grace, Lady Keepsake, Dowager Duchess of Keepsake, near The Chalk, is another ferocious and overbearing harridan in the list of Pratchett characters that includes Daphne's Grandmother and the plebeian but hardly humble Miss Maccalariat. She is snobbish, demanding and arrogant and browbeats everyone around her below the rank of King, including her daughter Letitia. Letitia is about to marry the young Baron of the Chalk, whom she also tries to browbeat, with less success than usual. Her attempts to subject Tiffany Aching to her will have strong echoes of Lady Felmet trying to break the witches of Lancre, with much the same result.
There is a chink in her aristocratic armor, though; Mrs Proust, the city witch, recalls her from the days when she was a music-hall dancer in Ankh-Morpork, named Deirdre Parsley, noted for high kicks and kicks in general. The Duchess's manner changes abruptly when Mrs Proust arrives as the prospect of being exposed as a not-at-all respectable showgirl improves her personality markedly. She has told everyone around the hubward plains that her family perished in a terrible fire long ago. This could have been the Great Fire started by Broadman, but we don't really know what became of the Ankh-Morpork Parsleys. This also explains why all the books on heraldry, genealogy and Keepsake family history, that should by rights have been in the library at Keepsake Hall, have disappeared, she claims, to her rooms, so she can study them more conveniently.
On the other hand, Letitia points out that the reason there are two hundred and fifty servants at Keepsake Hall is that most of them are old and retired, or looking after their elderly predecessors. However severe the Duchess's discipline may have been during their employment, no faithful servant has ever been cast off due to old age or disability. Noblesse oblige may be pronounced "nobless obligay" in Ankh-Morpork's entertainment district, but it's sometimes understood.
in Snuff, we learn she is known to Lady Sybil Ramkin, who, on 'behalf' of the Duke of Ankh (without his knowledge), has accepted a dinner-and-ball invitation to Keepsake Hall at which the Duke will be expected to dress appropriately to his station and office. No doubt, Sybil knows the secret of Deirdre Parsely, but can be expected to be discreetly charitable and behave appropriately to a fellow Duchess. Like Tiffany, the Duke may also be reliably counted on to make telling comment about the way she treats the hired help. Maybe as Commander of the AMCW, with the collective memory of the Watch to draw on, he also knows the secret of Deirdre's origins.