(adapted from the wikipedia entry)
The Red Court of Vampires are less human-like than the White Court. Instead, they are slimy bat-like creatures that wear "flesh masks," sensuous and beautiful human costumes that allow them to hide in plain sight. They are known for their seductive "Kiss". Their saliva contains a potent magical narcotic which addicts humans to their control very rapidly. The narcotic lowers the victim's inhibitions while dulling the pain of the vampire's bite. Addicted humans will go to great lengths to protect their vampiric masters. Red Court vampires are incredibly strong and fast, and they can shake off most injuries quickly unless their bellies are cut and their blood is spilled, which will kill them. The Red Court is also vulnerable to sunlight, which burns them, and are repelled by articles of faith.
The Red Court is also capable of transforming ordinary humans into vampires in a two-step process: the human is first infected with the intense vampiric thirst for blood (gaining supernatural speed, strength and endurance in the process) and the change into the full demonic form is completed upon draining a human victim in their first feeding. Infected humans with sufficient strength of will have been known to refrain from feeding for an indefinite length of time (magical bonds, regular exposure to sunlight, and avoiding physical intimacy all help), but no cure has yet been found for the "half-vampire" infected state.
The Red Court is highly organized in a feudalistic caste system with a King at the top, followed by Dukes and other ranks. As of Grave Peril, the Red Court and the White Council are at war.
She runs The Velvet Room, a high-priced escort service, for which she "kept a flock of beautiful, charming, and witty women, pandering them to the richest men in the area for hundreds of dollars an hour." The business is run from an enormous mansion that doubles as the setting for her great ball in Grave Peril, upon her ascension to the rank of Margravine.
She is killed in Grave Peril by the re-empowered ghosts of her victims. This event sets off the war between the Red Court and the White Council.
Duke Paolo Ortega
Duke Paolo Ortega is a warlord of the Red Court. He first appears in Grave Peril at Bianca's side, as a representative of the Red King. He compliments Harry on his ducky boxers.
He appears again in Death Masks, wherein he challenges Harry to a duel. He attempts to recruit Lord Raith as his second, but Raith sends his son Thomas instead as an insult. In the actual duel, Ortega cheats and nearly kills Harry, but quick action by Thomas, Susan, and Martin saves him. Ortega flees to his home in Casaverde, which Ebenezar later destroys, killing him and all the other Red Court vampires in the area.
He is survived by his widow, who was the highest bidder on the Harry Dresden eBay auction in Proven Guilty.
The Red King
The Red King is the highest noble of the Red Court, but we know little about him. He supports the war between the Red Court and the White Council, and he was present at a battle in Oregon during "Proven Guilty," in which Morgan got within 20 feet of him.