Beth Wynne, aka Karla (Bree Williamson), is a villainess from "Backstabber," episode 8.18 of Castle (airdate April 18, 2016).
Beth is a longtime grifter and con artist who was shown in the beginning of the episode enticing office worker Ned Werner, doing so under her alias, Karla. Ned had attempted to impress Karla by logging into the company's closed server and temporarily shut down the power at 10 Downing Street. After Ned left the office, the evil Beth hacked into the server, doing so by using a worm program that Hayley Shipton helped create, which was part of a higher-up's sinister plan to set up Hayley.
After Hayley's co-horts, Marcus Janek and Bryce Robert were murdered, an investigation led police to Beth/Karla, after Ned revealed his dalliance with her. Castle's knowledge of corsets led to Beth's reveal as a con artist, and later on, Beth was found at a motel and handcuffed by Ryan. She admitted to her role in the hack, but she denied committing the murders. It was later confirmed that Beth did not kill Marcus and Bryce, but she was arrested for the crimes she did commit.