Brinker Hadley has two roles as a leader in A Separate Peace. Brinker is arguably the most conventional character in the novel, embracing the notions, values and expectations put onto young adults during WWII.
He is a leader of various groups at Devon and he intends to enlist early in the armed services. Brinker cannot wait to meet his future expectations but wants to rush out to fulfill them as soon as possible.
In these ways, Brinker is a leader for many of the boys at Devon, including Gene, setting an example of conduct and aspiration for the others. This is the second signficant way in which Brinker is a leader - he influences Gene.
When Brinker decides to enlist early, Gene makes the same choice. Gene's respect for Brinker is clear.
Additionally, Brinker plays a central part in assembling the "trial" where Finny's accident is probed. Brinker organizes the trial and arranges to have Finny and Gene brought in for questioning. In this episode, Brinker is the sole leader and acts as both attorney and judge, orchestrating the event.