Remus John Lupin is a fictional character in a series of books about Harry Potter, written by J. K. Rowling. It first appears in" Harry Potter and the Prisoner of Azkaban " as the new Defense Against the Dark Arts Professor. Lupin remains in history long after he left this post as a friend of the main character. Harry Potter. In the films, he is portrayed as an adult by David Thewlis and a teenager by James Urchin.