The Man in the Red Sweater is unnamed person who Buck remembers for the rest of his life because this is the person who teaches Buck the lesson that "a man with a club was a lawgiver, a master to be obeyed."
Red Sweater firstly appeared when he breaks Buck out of his crate and when Buck attacks him, Red Sweater used with several times to stop Buck attacks. And when Buck calm downed, Red Sweater brought Buck food and started to say that if Buck act nicely everything is going ok, but if Buck is bad dog, Red Sweater started say that he is going to kill Buck. Later Red Sweater sold Buck and other dog Curly to French Canadian Perrault. This was last time when Buck ever saw Red Sweater.
Red Sweater was mention several times during book, when Buck remember that this was man who beated him for the first time.
Red Sweater once again was mentioned. When Francois walked to Buck with whip, he remembered how Red Sweater beat him.