Lady Roberta Meserole, also called Madam, appears in the book Night Watch. She is a very mysterious character who prefers to pull strings in the background. Not much is known about her. She claims to be from Genua, and it is unclear whether she has been married or not. She appears to have a liking for cats, albeit not long-haired white ones (a pointed reference to the James Bond Villain Stavros Blofeld), but preferring elderly street toms with irregular bouts of flatulence.
Lady Roberta is the aunt of Havelock Vetinari and although indirectly referenced in several other books, only makes a direct appearance in Night Watch. She played an important political role in the intrigue to overthrow Lord Winder, with the help of Vetinari, a student Assassin at the time.
She has sharp reflexes and the ability to throw a good punch: in Night Watch, she has an encounter with the young Nobby Nobbs, in which Nobby's advanced pickpocketing abilities prove no match for her discovery of his hand in her purse. She then shows great acumen: when Nobby regains consciousness, she realises his unique skills could be of use to her as a gofer, spy, and intelligence-gatherer. A less astute noble would simply have had all his fingers broken buried him indefinitely in the Tanty whereas Lady Roberta sees a rough-and-ready street asset. Nobby agrees immediately to this arrangement since she pays well and he has no desire to end up sharing a cell with his father. Lady Roberta is comfortable in high society and likes to flirt with politically important people, in order to further her goals. Friends call her Bobbi but whether she really considers anybody a close friend is debatable. Samuel Vimes seems to have earned her respect, and she secretly watches his moves and actions during the Glorious Revolution of Treacle Mine Road.
The character of Lady Roberta Meserole, as the mysterious lady, has many parallels in fiction most notably in the spy intrigue genre in the character of Milady de Winter in Alexandre Dumas' "The Three Musketeers". Equally Pratchett might have the real-life Roundworld model for Milady de Winter in mind - Lucy, Countess of Carlisle who, during the British Civil Wars of the 1640s, used her privileged position as an aristocrat and lady-in-waiting to the Queen to secretly aid the rebellion against the Crown. Other Roundworld referencs might include Lola Montez, who started out as music hall dancer and occasional seamstress, but who manoeuvred her way up the ladder to become Queen of Bavaria; or actress Eva Peron, who famously and tunefully became the power behind the throne in Argentina. Lady Marcia Falkender deserves a mention too: at first merely personal secretary to an ambitious young member of parliament called Harold Wilson, she stayed with him through his rise to Prime Minister. During his last term of office in the 1970's, there were rumours that as Wilson was diagnosed with Alzheimers and his role in politics consequently became shakier, it was actually she who was dictating and steering British government policy. There is also the interesting example of the real-world Marquise de Monnier, an unconventional and convention-defying woman who was active in Paris as a fixer and arranger and intermediary, in the years immediately preceding the French Revolution. She was mistress to the Compte de Mirabeau, an aristocrat who not only dodged the guillotine but became an influential figure (as opposed to a leader) in the French Revolutionary government. "de Monnier" wasn't her real name: she operated under a pseudonym and consequently had an aura of mystery and intrigue about her like all the other figures including Lady Roberta Meserole.
Meserole evidently is slang for a loud obnoxious drunken family group of family that teases and/or fights with each other.