Principal Lang is the principal of Danville High School. He is seen in the episode Doof 101. He seems distrustful of having Dr. Doofenshmirtz to work in his school as a teacher. It is later revealed that he was an old college rival of Doofenshmirtz during the '80s, and was jealous when Doofenshmirtz had won the heart of Charlene at that time. Even when being told that both Doof and Charlene have been divorced, Lang refuses to abandon his grudge against Doofenshmirtz.
He essentially takes on the role of Candace in the episode, attempting to use his grudge to "bust" Doofenshmirtz and fails, similar to how Candace fails to show her mother what Phineas and Ferb are doing.