The evil Emperor Palpatine pops up a few times in The Empire Strikes Back, but he remains a vague, other-worldly figure, leaving the muscleman Darth Vader to take care of the dirty work.
This is actually his first appearance in the original trilogy, although we did see him in the prequels as he transformed from the respectable Senator Palpatine to the fearsome Darth Sidious, corrupting a young Anakin Skywalker along the way. Based on the Emperor's brief interaction with Vader in this film, it's clear that he continues to hold a great deal of power over his apprentice:
VADER: If he could be turned, he would become a powerful ally.
EMPEROR: Yes, he would be a great asset. Can it be done?
VADER: He will join us or die, my master.
Interestingly, however, we start to see a few cracks in this relationship by the end of the movie. Just think about Vader's suggestion that he and Luke "destroy the Emperor" and "rule the galaxy as father and son," a.k.a. the Sith equivalent of playing catch. While you might argue that Vader is just playing mind games here, we see it as a clear sign that the baddest partnership in the galaxy isn't as strong as it appears.