Carthia van Canten, also known as "Cantarella", was the mistress of Vattier de Rideaux, the head of Nilfgaardian intelligence for Emhyr var Emreis. She was informing Assire var Anahid about Vattier's intentions. She had an excellent memory, a lively intelligence, and a mercurial wit.
During the event she offers Geralt a deal in which they will split the winnings. It turns out her participation in the tournament was a cover to allow her to gain enough money to travel back to Nilfgaard after her cover is blown.