Will Mayes is the guy we are rooting for in this tragic tale. We don't get many details of his life – but there is nothing in the story to suggest that he's guilty of any crime, or that he is a bad person. The fact that his ultimate fate is omitted from the story leaves us feeling uncertain and sad.
Hawkshaw is an important character, and the one character we see actively trying to do good in the story. Unfortunately he fails. Either from cowardice, lack of imagination, or fear, he jumps ship, or car, before his potential to save Will is put to the test. Remember, when Will jumps, Hawkshaw has already become complicit in Will's abuse. He helped restrain him and get him in the car. He probably fears that if he continues on the desperate journey he'll be forced to participate in Will's further abuse, and probably murder, or lose his own life. He's no saint, but relatively speaking at least, he qualifies for protagonist status.
For all her flaws, faults, and potential complicity in Will's ultimate fate, Minnie captures the sympathy of many readers. Such readers see her as a victim of the society in which she lives, and want something better for. She can be placed in the protagonist category because want to see her move from the desperate position she occupies in the story.