Jealousy depends on what is perceived by one person about another and then compared to one's self at an unachievable higher standard. In a growing up story like "A Separate Peace" many characters perceive themselves as the underachiever in the group when in fact they simply have different strengths and weaknesses. This is common among teens as they are discovering who they are and how they fit within their peer groups. Since the book is written from Gene's perception, we mostly see his jealousy towards Finny. One of the first tings Gene notices is, "I never saw him tired, never really winded, never overcharged and never restless" (39). Here are a few more to look at:
"I found it. I found a single sustaining thought. The thought was, You and Phineas are even already. You are even in enmity You ae both coldly driving ahead for yourselves alone" (53). This one is a realization of selfishness based on previous feelings on jealousy.
"You did hate him for breakig that school swimming record, but so what? He hated you for getting an A in every course but one last term" (53).
"Finny had deliberately set out to wreck my studies," suggests that Finny was jealous of Gene (53).
In the end, after they've matured a bit more, Gene describes his jealousy like this: "No, I don't know how to show you, how can I show you, Finny? Tell me how to show you. It was just some ignorance inside me, some crazy thing inside me, something blind, that's all it was" (191).