In A Separate Peace, Phineas (Finny) is the best athlete in the school and most of the students and teachers find him so charming that he seems to be able to do whatever he wants. In a school where boys are being groomed to become men (and sent off to wars), Finny does not, and seemingly is not expected to, accept the adjustment into young adulthood. This is the other part of Finny's outlook or his nature. One could argue that Finny is actively trying to stay young at heart, or one could argue that his nature simply is to be young at heart.
To whatever degree Gene caused Finny's fall, and ultimately his death, Finny and Gene were friends; not enemies. Gene realizes later that he was not jealous of Finny's talent; he was jealous of Finny's innocence and his (Finny's) reluctance to grow up and accept the reality/futility of war. Finny symbolizes the opposite of the upcoming war. Finny was innocent, playful and without a shred of harmful intentions. It is ironic that Finny, the most innocent character, ends up being injured (and killed) without seeing a single battle in the war. The accident is perhaps a tragic reminder that innocence cannot endure in a world where war is still a common means of solving disputes.