While living in the woods, Sam befriends several animals who live in the area. He and the animals maintain a healthy respect for each other, since Sam knows that wild animals can be dangerous and the animals understand how humans catch and kill them. One of these friends is a weasel who lives behind a large boulder near Sam's hollow tree. The weasel is not afraid of Sam, but playful, and with the others, gives Sam an outlet for conversation so he can keep his sanity. Sam calls the weasel "Baron" because he is dignified and protective of his habitat; the weasel also seems to live in harmony with the other animals, and because of the large boulder, almost has a protecting castle over his den.
...he darted right to my knee, put his broad furry paws on my pants, and looked be right in the face. I shall never forget the fear and wonder that I felt at the bravery of that weasel.
Presently, down he climbed, as stately as royalty... And so the Baron and I met for the first time, and it was the beginning of a harassing but wonderful friendship.
(George, My Side of the Mountain, Amazon.com)
Some of the other characters meet the Baron Weasel and are enamored with his unafraid and playful nature. Sam also views the weasel as a sort of monitor for the safety of the woods; when the Baron hides, Sam knows that there is something either dangerous or unusual around that his human sense can't perceive.