Nadja is a villainess from "The Devil's Ticket," episode 1.29 of Thriller (airdate April 18, 1961).
She was played by Patricia Medina.
Nadja is the mistress of artist (and main protagonist) Hector Vane, and regarding a backstory, it was only revealed that Nadja was a model for Hector at some point before their illicit affair began, and it was in the episode's midpoint where Hector and Nadja were shown in a romantic embrace. Nadja later mentioned plans for a trip to Mexico and coerced Hector to join her, only for Hector to learn that the date of their trip was just days short of his deadline with Satan.
In the episode, Hector's attempt to pawn one of his old paintings saw him encounter Satan himself, leading to Hector pawning his soul for three months, but he would be spared if he could paint a human soul during that exact time frame. In addition, Nadja turned heel when she was enlisted by Satan into tempting Hector away from his goal, with the trip being part of the plan. The episode's progression saw the villainess become enraged by Hector's indecision, with her anger increased when Hector originally turned her down. Hector decided to paint a portrait of his wife, Marie, as part of the deal, and later showed it to Nadja while stating that he could go with her on their trip.
However, upon seeing the portrait of Marie, the evil Nadja grabbed a knife and destroyed it in a psychotic rage, as she saw the portrait as a symbol of Hector's devotion to his wife. The episode's climax saw Hector deliver a replacement portrait to Satan--one of Satan himself, with the climax also seeing Hector correctly deduce that Nadja was a villainess sent by Satan to tempt Hector. However, Hector was unable to provide the pawn ticket he needed for to redeem his soul, due to the fact that Marie burned Hector's old coat (the ticket was in one of the pockets). While Hector ended up dragged to Hell, Nadja's fate wasn't revealed.