Brinker Hadley is that guy. You know, that guy: the one who dresses well, looks good, heads up a few dozen committees, speaks with ease, and always smells really, really good. Brinker is a natural born leader and used to being in charge. But of course, this is A Separate Peace, and change is always in the air. Brinker's transition from a model student to a reticent rebel is the meat of his character.
We talk about this more in Character Role ID, but Brinker and Finny form one of many dichotomous pairs in the novel. Summer/Winter, youth/adulthood, anarchy/laws, peace/war. (Brinker's persistent accusations against Gene reflect his need for order and his tendency toward positions of authority.) The transition from the Summer to Winter session means Finny leaves the seat of power and Brinker enters it – but only temporarily. When Finny comes back to Devon, we expect a showdown of sorts, and Knowles delivers, via the Winter Carnival. The battle is brief, and Phineas wins out. Or, as Gene says, "Brinker the Lawgiver had turned rebel for the Duration" (9.39).
But it's not until the end of the novel that Finny realizes what initiated Brinker's transformation in the first place. Like much else in A Separate Peace, this has everything to do with the war. Brinker, who everyone thought would be the first to enlist, quickly becomes disillusioned with the idea of fighting. Gene identifies his feelings as "a faintly self-pitying resentment against millions of people he did not know" (13.40). Brinker doesn't want to fight in the war his father started.
This is interesting. Initially, Brinker represents responsibility and adulthood. He appears to be more mature than the other boys, and his acceptance of the war and eagerness to enlist would seem to reflect that. But by the end of the novel we realize that Brinker's abandonment of his earlier ideals represents his growth. His maturity is embodied not by embracing the war, but by rejecting it. This puts a twist on the youth/adult, peace/war, anarchy/rules dichotomy that has thus far dominated the novel.