Lady Margolotta is a wealthy noblewoman and a Vampire; a founding member of the Überwald League of Temperance or Black Ribboner, who has sworn off drinking blood. Black Ribboners "switch" their craving for blood to crave something else which, in Lady Margolotta's case is control over people. She was a patron of William de Worde's newsletter before he founded the Ankh-Morpork Times and she maintains many such sources around the Disc. Having a Vampire's power of persuasion and attention to detail, she stays on top of the political situation throughout Disworld, mainly across Überwald and the Sto Plains, influencing it subtly and slowly since Vampires have lots of time. She is therefore a leading political influence across the empires, nations and city-states of the Discworld. Lord Vetinari, Ankh-Morpork's current Patrician, spent some time with her during his Grand Sneer (and escape from Snapcase's Ankh-Morpork) apparently laying most of the groundwork for his own rise to power with her. Margolotta claims to have learned a lot, especially about manipulation, from Vetinari who had graduated from the Assassins' Guild some time before starting on his trip. (although there is some debate about whether he graduated before or after) and the two have maintained a relationship since those days, playing Thud against each other via the Clacks. Some indications are that their relationship is a romantic one.
She appears to be an older and distinguished vampire, having 4 pages in the Almanack de Gothic. She sports a pink jumper with embroidered bats in The Fifth Elephant, and in Unseen Academicals Nutt mentions that she also owns a dress made largely of leather and rubber. Her coach is entirely black, with black glass in the windows, black horses with black plumes, and no driver. Samuel Vimes also noted that she "looked like someone's mother".
She lives with her small rat like dog in a castle near Bonk, in Überwald. Her castle's architecture is described as "being influenced by fairytales and the more decorative form of cake" and could be a parody of Roundworld's Neuschwanstein Castle, (the building of which drove a King of Bavaria to insanity and bankruptcy) all though there are many castles that fit the bill.
Lady Margolotta appears in The Fifth Elephant, Unseen Academicals, and Raising Steam besides being casually mentioned by Vetinari several times in the novels between.