In Chapters 5, 6, and 7 of A Separate Peace, who thought Gene and Finny were competitors?
Besides Gene (who sees Finny as his competitor until Gene realizes that he is in competition with himself), Brinker always assumed that Finny and Gene were competitors. Brinker's assumption is natural, because he is himself in competition with the rest of the boys (especially academically). He tells Gene when Gene returns to Devon after break,
" 'I'll bet you knew all the time Finny wouldn't be back this fall. That's why you picked him for a roommate, right?' " (88, Chapter 7).
He insinuates that Gene is secretly happy about Finny's accident because it has eliminated his competitor. Brinker does not know, of course, at this point that Gene did deliberately hurt Finny. So, his comments are darkly ironic.
In regards to Gene's competition with Finny, after he hurts Finny, he realizes that he must do whatever he can to make up for it. He even becomes manager of the crew team so that he can tell Finny that he's involved in athletics. Gene makes this decision when he visits Finny in Boston and sees that he has become "an invalid, house-bound." He knows that he is responsible for the new version of Finny and begins to see him not as his competition but rather as his mission.