Brinker spends the entire story trying to make the best possible situation for himself and taking offense at anyone else who has success that he is unable to duplicate.
The first time Brinker enters the story, he comes into Gene's room and congratulates him on achieving a room without a roommate. The fact that Gene did not request a single room never occurred to Brinker, and his insinuations that Gene requested to room with Finny again while conscious that Finny would not be returning to Devon convey his jealous.
When Brinker first becomes frustrated with the war efforts demanded of the Devon boys and announces his plan to enlist, he sees this as a plan that will allow him to escape the madness of the school and make a glorious name for himself. When he discovers Finny has returned to school and realizes that Finny and Gene would be continuing on to complete their careers at Devon before entering the war, Brinker has to remain and complete his time, also.
As the story ends, Brinker enlists in the Coast Guard because he sees it as the safest and easiest way to fulfill the military obligation he knew he had to face. Brinker has no understanding or respect for his father's opinions regarding acceptable attitude and intentions toward becoming involved in the military forces; his primary emotion os "generalized, faintly self-pitying resentment against millions of people he did not know."