What do you mean, "BESIDES" defining the judicial branch? That's what John Marshall was all about! He was a Federalist judge who remained in his position handing down Federalist rulings for decades after the Federalists were gone. He was involved in some of the most important court rulings in the history of this country, rulings whose effects are still felt today. Read up on him - you can write an entire book about just about any one of his major decisions, so it shouldn't be too hard to write an essay on his overall impact.
http://en.wikipedia.org/wiki/John_Marshall