How does Brinker impact the other boys at Devon, and how does he make them feel more "complete"?
Brinker impacts the other boys at Devon in a way that is completely opposite to the way that Finny impacts that boys. While Finny is spontaneous and free spirited, Brinker is more reserved. While Finny finds ways to go around the letter of the law, Brinker is much more interested in following the letter of the law and having justice be served. You could even illustrate Finny's and Brinker's dichotomy with the seasons. Finny is in control of the boys during the Summer session at Devon. Brinker is the leader during the Winter session at Devon.
His impact on the boys is through his sense of justice. He has a strong moral compass and guides the boys in that regard. Sometimes his moral compass and desire for justice is a bad thing though. He is the boy that leads the case against Gene, which ends up destroying a friendship and ultimately leads to Finny's death. He also impacts the boys with his view of the war. When the novel begins, all of the boys assume that Brinker will be the first to join the military and go to war. But as the novel progresses and Brinker matures, his view of the war becomes much more jaded and pessimistic. That in turn affects how the other boys view the war as well.
As for how he makes the boys feel more complete, I believe he does that by acting and being a strong, rule following leader. Without Brinker the group is a loosely held together group of boys. They are friends, but Brinker brings a sense of purpose and direction to the boys. If you think of the group like a single body, Brinker completes the body by being the head. Finny would be the heart, but Brinker is most definitely the head.