Cindy Stoker is the wife of Spike Stoker and mother of Betty Stoker. In Lost in a Good Book, Spike mentioned her as his girlfriend, and when Thursday Next returned to Swindon in Something Rotten, they were married.
In Something Rotten, it was revealed that Cindy was in fact the assassin the Windowmaker, which Spike had no idea about (she told him she was a librarian), and she had been hired to kill Thursday. She tried and failed three times, the last ending with a grand piano landing on her head. She was knocked into a coma.
She then saved Thursday's life when Thursday was shot during the Superhoop by volunteering to take her place, as she would have survived her injuries.