Who was the man in the Red Sweater in Call of the Wild?

The man in the red sweater is a dog-breaker who teaches Buck the Law of Club and Fang. 

Buck lives a very comfortable lifestyle on Judge Miller’s ranch.  He is a pampered pet.  Yet when he is stolen by Manuel, he is not quite ready to be a sled dog.  He still believes that his word is law. The man in the red sweater teaches him to obey the person with the club. 

A stout man, with a red sweater that sagged generously at the neck, came out and signed the book for the driver. That was the man, Buck divined, the next tormentor, and he hurled himself savagely against the bars. The man smiled grimly, and brought a hatchet and a club. (Ch. 1) 

This man’s only job is to “break” Buck.  In other words, he will hit him with a club enough times that Buck will stop fighting.  From then on, Buck understands that you do what a man with a club says.  By extension, he soon also learns that he needs to obey stronger dogs (that is the “fang” part of the law).  Buck puts up a fight, but he eventually learns it is futile to disobey a man with a club. 

For the last time he rushed. The man struck the shrewd blow he had purposely withheld for so long, and Buck crumpled up and went down, knocked utterly senseless.

"He's no slouch at dog-breakin', that's wot I say," one of the men on the wall cried enthusiastically. (Ch. 1) 

It does not take Buck long to learn the ways of the wild.  In the wild, you do what you are told, whether it is by the man or the dog.  Being on a sled-dog team is serious business.  If a dog doesn’t listen, either to the lead dog or to the human in charge, there will be consequences.  This is the Law of Club and Fang.