Which character’s actions ultimately lead to Buck's hardships on the trail?

The character whose actions lead to Buck's hardships on the trail is Hal. Hal represents the brutal and careless attitudes of inexperienced trail bosses who prioritize speed and profit over the welfare of the dogs. When Hal, along with Mercedes and Charles, takes charge of the sled team, he exhibits a lack of understanding and care for the animals, pushing them too hard without proper consideration for their limits.

Hal's insistence on overloading the sled and driving the team relentlessly leads to severe exhaustion and suffering among the dogs. This disregard for Buck's well-being and the harsh conditions they face contributes significantly to the hardships that Buck must endure. Therefore, Hal's brazen management and lack of empathy become the direct cause of Buck's struggles on the trail, showcasing a clear example of the consequences of neglect and brutality in a survival situation.