Brian changes fundamentally in the course of this story. The Brian in the first part of the book—before the rescue plane passes overhead and leaves him behind—is an ordinary kid, smart and resourceful but a little tentative, a little unsure of himself. The Brian of the latter part of the novel is altogether different, a kid whose reaction to a moose attack and a tornado is basically to say, "Bring it on!"
Having sunk to the very bottom of despair and found his way back up to the light, Brian has developed "tough hope" (13.37). And this very determination, more than anything, is what allows him to keep going and to survive. The two Brians of the novel basically present two different ways of coping with adversity, and it's only by thinking of the "old" Brian that we can see how far the "new" Brian has come.