Charlie is a Border Collie and is a black tricolor. Due to his breed, the people of MacFarlane's Ranch most likely use him to herd the cattle and sheep when not guarding the ranch at night.
Charlie accompanies Marston in the MacFarlane's RanchNightwatch missions. During the missions, he will occasionally attack anyone involved in criminal activity at the ranch . This can make subduing a criminal hard sometimes, as they may be mauled by the dog.
If the player whistles while near Charlie, Charlie will follow the player if they leave the ranch and point out and attack animals in the wild.
Charlie has a large amount of health, making it hard for any outlaw to kill him.
Sometime in the MacFarlane's Ranch, Charlie can be seen fetching a stick with a man or woman.