Most of the boys at Devon are willing to live with the mystery of what happened to Finny on that fateful day when he fell from the tree. But not Brinker. A stickler for the facts, he wants to get to the bottom of the matter. He suspects Gene had something to do with Finny's accident, and so he convenes an impromptu court to try him for his alleged crime.

As previous educators have rightly pointed out, Brinker is the kind of guy who likes rules, order, and stability. It's part of his personality. And the onset of war has, if anything, made him even more appreciative of the importance of these values.

Brinker's one of those people who always likes to be in control. Yet now, in the shape of World War II, he's confronted with a cataclysmic event over which he has no control at all. Setting up the trial and effectively arresting Gene is his way, then, of getting some measure of control back into his life. There's nothing he can do about the increasingly dangerous, scary world outside Devon, but within the confines of the school, he's still kind of a big deal; his actions still matter. Here, he still enjoys control over events.

When Brinker becomes the leader of the boys, he acts very differently than Phineas had. In fact, Brinker is more logical and rational, and he simply wants to determine the truth about everything. Because of this, he is very curious about the injury to Finny, and he believes that Leper witnessed the incident. Brinker is a very conscientious and rule-following boy, so he doesn’t want there to be any hidden wrongdoing.

Therefore, he holds the trial to determine what actually happened the night Finny fell out of the tree, and he has suspicions that another boy, Gene, was up to no good. Due to his curiosity and law-abiding nature, along with his doubts about Gene’s innocence, he is compelled to create a court of Gene and the Cohorts to determine what actually happened that night—because he values the truth and justice so highly.

When Finny returns to campus, Brinker insists on a trial because he suspects that the story being told about Finny's fall as an accident is not correct. Brinker is a law and order person who likes clearcut lines of authority. A trial, which represents truth and justice, is just the kind of thing Brinker likes to rely on. He is a facts man.

He is also a rival of Finny's for leadership in the school and identifies as a close friend of Gene's. There seems to be some malice in the ruthless way he goes about this trial, catching Gene and Finny by surprise and bringing them to face the "charges." Brinker is not without motivation to want to break up this close friendship, especially as he sees Finny as a rival for Gene.

Brinker believes Gene probably wobbled the tree branch, and he believes that with Leper's testimony, he can expose the truth that he feels both Gene and Finny are hiding.

Of course, it all backfires horribly.

Brinker Hadley is the antithesis of Phineas, law-abiding and conservative in his views. When Finny leaves school for a while, Brinker moves into the position of leadership, but more as an authoritative figure. Disillusioned with the idea of war as glorious after learning some things and witnessing the breakdown of Leper, Brinker desires the truth about other matters. He knows that Leper was a witness on the night of Finny's injury, and when he learns that Leper is on campus, Brinker has one of the boys fetch him.

But, before Leper arrives, Brinker clearly suspects Gene of foul play against Finny. Cautiously, he asks,

"...it wouldn't do you any harm, you know, if everything about Finny's accident was cleared up and forgotten." 

Later, "Brinker and three cohorts" came charging into Gene and Finny's room in order to take them to the Assembly Room where Brinker speaks of "inquiry" and "blame on the responsible person." Finally, Brinker answers Gene's question of why they are there: "Investigating Finny's accident!"

"After all," Brinker continued, "there is a war on. Here's one soldier our side has already lost. We've got to find out what happened."

Gene becomes shaky, saying he does not think it a good idea, but Brinker cuts him off, telling him it is being done for Finny's good and Gene's as well. Clearly, he wants Finny to know that Gene's act on the tree bough was one of treachery. Clearly, Brinker is suspicious of Gene, and Leper's testimony indicates that the two friends were on the bough at the same time "like pistons" and when one moved down, the other moved up. But, Leper refuses to implicate anyone. Finny, however, knows; startled by the truth, he hurries from the room and falls clumsily down the marble stairs.