Nature, Time, and Physical Reality
As far as antagonists go, this is a pretty big one. If you think of Nature as all butterflies and daisies, you're definitely in for a surprise here. Nature in this story is neither tame nor friendly; it's got a lot of weapons in its arsenal, and it's not afraid to use them. The nasty things Brian has to deal with include aggressive animals, injuries, hunger, extreme temperatures, and "act-of-God" type weather events.
Throughout the book, he's in a race to see if he can stay ahead of the powers of Nature long enough to avoid starving, getting mauled, or otherwise meeting an early and unpleasant demise. The fact that Brian's antagonist isn't actually, consciously trying to hurt him—that it in fact doesn't care whether it hurts him or not—somehow only makes it that much worse.