Ennis Del Mar
We open and close with Ennis, which makes him a bit more of a protagonist than Jack. Though to be fair, they're both pretty influential when it comes to the plot. The story is theirs, the problems are theirs, and we ultimately care about the two of them far more than any of the other characters. That's pretty firm protagonist territory, and since (1) Proulx follows Ennis around a lot more than Jack and (2) Ennis is not, in fact, dead at the end, Ennis gets the pole position.