Spitz was the main antagonist during the first three chapters. He is Buck's arch enemy and the leader of the team.
Going to Yukon
Spitz first was introducted in ship along with other sled dog Dave.
Enemy with Buck
After watching how Curly died from huskies attack, Spitz laughed at her fate and from that minute on, he became Buck's archnemesis because Buck hated the fact that he laughed at his friend's death.
When Spitz causes the chase of the rabbit to come to a halt, and the rabbit is ripped apart and eaten, Buck (who led the chase) knows that it is time for the final battle for mastery with Spitz. At first Buck is severly wounded and bleeding from many cuts all over his body, while Spitz is untouched. The fight gets desperate. But eventually Buck breaks Spitz's two fore legs and Spitz is left helpless, trying to drive away impending death by snarling and bristling. But Buck injures him even more, leaving the remainder of poor Spitz to be torn apart by the other dogs who watched and waited all around them. That is the last Buck saw of Spitz. He vanishes from sight under the mass of sled-dogs. Buck rejoices over his victory and claims leadership the following day.