Starbuck is Ahab’s right-hand man, but he’s also an important contrast to Ahab. Captain Ahab is all monomaniacal fixation: there’s a single object on which he’s pinned all his rage for vengeance, and he’s going to use any means that come to hand in order to destroy it.
As Melville reminds us, Ahab’s methods are sane, but his motive is crazy. Despite his semi-successful mask of sanity, though, Ahab is an intense figure almost too ready to take action; even his interminable watches on the deck of the ship show how he just can’t stop doing things.
In contrast, Starbuck, his first mate, has a moderate, ethical objective to stop Ahab’s perverse craziness, but he can’t bring himself to use any methods to bring it about. So, Starbuck stands around doing nothing and being right, while Ahab marches around doing a great deal and being oh so very wrong.
Captain Boomer, the English captain on the Samuel Enderby who lost his arm to Moby Dick, is only in the novel for one chapter, but in that time he becomes a crucial contrast to mad Captain Ahab.
Even though Boomer suffers as a result of Moby Dick’s behavior in almost exactly the same way that Ahab has, he reacts completely differently, concluding that the White Whale is best left alone. He’s also able to step back and realize that a whale is not actually malicious. Appearing toward the end of the novel, Boomer becomes a momentary foil to Ahab, reminding us, just before we come to the climax, that it didn’t have to be this way.