Who is Deer from Brian's Winter and what is their importance?

Brian begins to think in terms of larger prey when he salvages meat from a deer killed by wolves. Later Brian will kill a buck himself, taking from it everything he could use. Brian feels guilty about both kills, leaving the head of the original deer in a tree so it can overlook the woods, while apologizing to both deer. Brian can only assuage his guilt by telling himself that everyone needs to eat, humans and deer alike.