Antagonist

The antagonist keeps the film going. The antagonist doesn't necessarily need to be a "bad guy", but he has to be a strong obstacle for the protagonist's change.

The Star Wars have a traditional antagonist. Without Darth Vader there would be no story. Darth Vader is the obstacle that Luke Skywalker must win in order to reach his goal. After the victory Luke will get "the princess and half the kingdom". Or, in this case, rather, half the universe.