Madwa Korel is a rogue Smoke Knight loyal to Lucrezia aka the Other. She has worked with (among others) Doctor Dio Zardeliv, Ariadne Steelgarter, and Doctor Francisia Monahan. She is the only Smoke Knight to use poisons that include Ghost Spider venom.
Madwa is a tall and very slender, even skinny, woman. She has pale skin, light-gray (almost white hair) and dark eyes. While appearing to be of rather advanced age, she seems quite physically and mentally fit. She wears the same purple and black colors favored by other Smoke Knights; but, unlike the others we have seen so far, has a diamond-shaped mark on her forehead; this adornment appears to be black at first, but is red in later appearances. It may be shared by all members of her group and it may be permanent, given that when she's first introduced we also meet "Bartleby" who was presumably a member of her group but is now an incinerated corpse who still displays this mark on his/its forehead.
Madwa was kicked out of the Smoke Knights by Grandmother, due to her continued loyalty to Lucrezia/the Other after all the Knights were ordered to cut their ties to Lucrezia or anyone loyal to her. Madwa and her group had been slavishly loyal to Lucrezia, however, and refused Grandmother's ultimatum, staying allied with her and the Geisterdamen.
Agatha first encounters her and Dr. Zardeliv in the catacombs under Paris, where they are all searching for Prende's Chronometric Lantern. Madwa soon shows her advanced Smoke Knight skills, evading a gas attack (that instantly fells Zardeliv), flitting about unseen, shaking off a serious electrical jolt, escaping the grasp of Prende herself and finally making off with the Lantern, albeit in a damaged state. In the process she appears to have one of her hands seriously burned or withered in some fashion by the revived Storm King.
She is next reported to be taking a submarine to England with Zardeliv still in tow, but Agatha and Co. are able to get there first via airship and are waiting when the sub comes crashing into Londinium's undersea docks. The sub is forced open to reveal everyone inside dead via gas-poisoning, including (pretty definitely) Zardeliv and.. an unidentified female whose body is conveniently kept off-stage. There is no sign of the Lantern.
Some time later and rather unsurprisingly, it turns out that Madwa is in fact Violetta is also still alive following their little melee in the Queen's Society dome, prompting Ariadne to flee Londinium. They travel together to Monahan's ocean lair, currently known by the fan-assigned named Big Rat Island. Madwa intends to kill the airship captain who brings them there, but his indifferent prediction of this very action causes her to her hand, at least for the moment. And speaking of hands, while the three women (in vain) to ambush Agatha and Co's investigating airship, we learn that Madwa's afflicted appendage has been fully healed or replaced or whatever., and actively scheming towards some unknown goal with both Ariadne Steelgarter and Francisia Monahan; she brings word to the former woman that
While the other two women work with a copy of Lucrezia to get the island's resident Mirror sort of up and working, Madwa patrols the perimeter, stretching herself thin in the process even with her various potions. Finally, using the Lantern, she covert intruder Hadrian Rakethorn even as he attempts to flee a very overt invasion force that has come pouring over the cliffs; these newcomers appear to be Deepdwellers who have been twisted by the contaminated waters being spewed under the island. Violetta is in the vicinity at the time, and fakes also being caught in the Lantern's light. This ruse works on the exhausted Madwa, allowing Violetta a chance to stab her in the back. Violetta snags the Lantern and hastily departs with a confused Rakethorn before the Deepdwellers get directly involved. Whether Madwa is 100% dead is again uncertain, but she's at least in pretty bad shape at the moment.