He is one of the professors at Harvard Law School. After Elle speaks up in his class, he offers her a chance at the internship during which he hits on Elle. Emmett and Brooke later got back at him for the harassment.
Professor Callahan had a bad ending at the end of the movie. He was not only fired by Harvard for harassing Elle, but his reputation as a lawyer is ruined, his ex-wife takes everything from him, and is sent to jail where he's roommates with Enrique.