Buck
The protagonist. A large, strong dog who starts the novel comfortable and ends it wild. His transformation is the entire story. Every chapter shows him shedding another layer of domestication and replacing it with something harder and more instinctive.
John Thornton
The one owner who genuinely cares about Buck. He saves Buck's life by cutting him free from Hal's doomed sled team. Buck's loyalty to him is the deepest human-animal bond in the novel. His death removes the last thing keeping Buck in the human world.
Spitz
The lead sled dog and Buck's main rival. Spitz is experienced, brutal, and dominant. His conflict with Buck builds across several chapters and ends in a fight to the death. Buck wins and takes his place as lead dog.
Hal, Charles, and Mercedes
Three incompetent and cruel owners who take over Buck's sled team mid-novel. They overload the sled, starve the dogs, and ignore every warning from experienced mushers. Their refusal to listen kills most of the team and eventually kills them.
François and Perrault
The first professional mushers Buck works for. They are tough but fair. They recognize Buck's ability and move him up the team. They're not warm, but they're competent, and under them Buck learns what real sled work demands.