What is the relationship between Faust and Mephistopheles?

In Faust, the title character sells his soul to the devil to gain all eartly knowledge. The devil, unable to fully control all of the situation sends Mephistopheles as kind of a mediator between what Faust wants and how the devil expects it to go down. If you've never seen it I highly recommend watching the movie from the 20s. Amazing film and a faithful adaptation. Directed by F.W. Murnau, as in the guy who directed the original Nosferatu film.