Buck, the hero of the story, a dog that is part German Shepherd and part Saint Bernard. Buck is abducted from his home in San Francisco to serve as a sled dog. He is abused cruelly by his first masters, but he adjusts admirably to the elemental life of the Yukon. He has to fight several dogs to maintain his place in the pack, and he is called on to perform several extraordinary feats of endurance and courage. He serves his human masters faithfully, but when his final master dies, Buck answers the call of the wild, forsaking civilization and joining a wolf pack, returning to an instinctual mode of life that always has been an incipient factor in his strength and indomitability.
Spitz, the malevolent head of the pack. He attacks Buck repeatedly, sensing Buck’s fitness to supplant him as the dogs’ leader. Spitz is valued by his masters for his ferocity and intelligence, but the masters concede Buck’s superiority when Buck challenges Spitz to a fight to the death and wins.
Perault, Buck’s first Yukon master, a hard but fair man who acknowledges Buck’s supremacy among the dogs. Perault and Buck drive themselves equally hard, and Perault makes the dogs an extension of his own will.
Francois, Perault’s French-Canadian partner, who predicts that Buck will become a great sled dog, perhaps the finest in the Yukon. He treats Buck with enormous respect and knows how to get the best out of the dog.
Hal, Buck’s next master, a crude man who ignores the advice of experienced Yukon travelers, whipping his dogs into a state of exhaustion. As the dogs weaken, he shoots them. He nearly kills the prostrate Buck when he refuses to respond to the whip, and only the intervention of John Thornton saves the dog. The reckless Hal perishes when his overloaded sled breaks through thin ice.
Mercedes, Hal’s sister. She accompanies Hal and their brother Charles on the foolhardy Yukon trip. She takes too many unnecessary things that weigh down the sled, and she interferes with the dogs, feeding them too much. Her passiveness and insistence on her helplessness contribute to dooming the journey.
Charles, Mercedes and Hal’s brother. He constantly quarrels with Hal as the two men fritter away their food supply and cruelly drive the dogs even when it is apparent that the team is sapped of all energy and of any will to continue.
John Thornton, Buck’s last master and his benefactor. Of all Buck’s masters, Thornton most obviously recognizes Buck’s greatness. He develops an intimate love for the dog that makes Buck perform incredible feats, including breaking out a half-ton sled, frozen in its tracks, and pulling it one hundred yards. Buck repays his master’s pride and confidence in him not only through his prodigious work but also through a fierce watchfulness. Only when Thornton is overwhelmed and murdered by a band of Indians does Buck feel released to the wild.