Jack Byrnes is a fictional character from the movie series "Meet the Parents" and its sequels. He is portrayed by actor Robert De Niro and is known for his intense personality and protective nature, especially towards his daughter, Pam Byrnes. Jack works as a former CIA operative, which adds to his intimidating presence. In the films, Jack's interactions with Greg Focker, played by Ben Stiller, create comedic tension as Greg tries to win Jack's approval. Jack's character is often depicted as overly cautious and suspicious, leading to humorous situations as Greg navigates his relationship with Pam and Jack's expectations.