For much of the story, Brinker represents the old-school, by-the-rules student who meets all the traditional expectations. He likes the power that comes with being a "leader" and is eager to pressure others into doing whatever gives him an advantage.
Brinker looked the standard preparatory school article in his gray gabardine suit with square, hand-sewn-looking jacket pickets, a conservative necktie, and dark brown cordovan shoes...He looked but happened not to be athletic, being too busy with politics, arrangements, and offices.
Because he thrives on exerting power over others and because he feels it is important to maintain the rules and proper procedures, Brinker is the instigator of both of Gene's trials - one in the Butt Room, the other in the Assembly Room of First Building. Brinker thinks he is enforcing the rules of conduct by one student toward another by accusing Gene of the crime; what he is actually doing is demonstrating how little perception he has regarding the emotions and circumstances others endure.
I had no idea what Brinker might say or do. Before he had always known and done whatever occurred to him because he was certain that whatever occurred to him was right.
As the story ends, Brinker has had a change of heart. He resigns from all his student government involvements and approaches the war involvement all the graduates must face with no personal enthusiasm. He no longer cares about pleasing his World War I veteran father's concepts of how they should approach what his father called "your greatest moment, greatest privilege, to serve your country."