Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
First Person (Central Narrator)/Bayard Sartoris
Bayard is the protagonist, because he's the one doing all the telling from his own point of view. The story is about his own, personal coming-of-age, and the different factors—like war, death, and revenge... you know, typical fare for any young kid—that make him the man he is today.
Speaking of the man he is today, the story is told from the adult Bayard's point of view, going way back to his childhood. That puts some elbow room between the telling and what gets told, which means we get some critical distance and Bayard is able to judge actions and events as an adult, even though he tries to remember what it was like as a kid.
The best examples of this way-back narration are when Bayard describes his father. Check out the way he talks about his dad seeming big to him when he was a twelve-year-old (we can tell that as an adult he realizes he wasn't all that big):
And that's what I mean: about his doing bigger things than he was... He stopped two steps below [Granny], with his head bared and his forehead held for her to touch her lips to, and the fact that Granny had to stoop a little now took nothing from the illusion of height and size which he wore for us at least. (1.1.26)
Bayard seems disillusioned: his father was a marvel to him as a kid—a giant!—but now he sees him as a defeated man who made grandiose gestures to seem bigger than he really was. And that's just one of the many amazing things that narrative distance can give you: it tells you how someone really feels about his dad without him having to spell it out.