The right answer is definitively Curly
The story starts with Buck spending a very comfortable life in his master´s house.
One day one of the gardener´s helpers, Manuel kidnaps Buck in order to pay his gambling debts, this is where Buck´s ordeal starts; he is beaten and thrown into a train heading for Seattle. Buck is then handled to his new handler. a man wearing a red sweater, who beats him with a club after Buck tried to defend himself, learning this way "The law club and fang"
Buck is then now sold to Perrault, who introduces him to his new life as a sled dog, where he meets characters who train him in the rules of being a sled dog which he learns.
However, Curvy who was a dog who was shipped with him in the way to Seattle was killed by vicious dogs before even getting any opportunity to get along with Buck.