Tools of Characterization
Gene lies a lot, and often to himself. He tells us that he's proud to be Finny's friend, that jealousy is OK and to be expected, that he's uncertain of what he really meant to do in the tree. Because we can't always trust his words, we have to look at his actions instead. Look at his actions against Phineas (the tree thing), but also his actions on Finny's behalf (training for the Olympics to appease Finny's dream). This is how we should judge Gene, although the fact that these actions are filtered through the memory and words of the narrator makes that job a little more difficult.
If we can trust Gene's words, his explicit descriptions of his classmates in A Separate Peace are the best way to understand these other characters. Check out his passages on Brinker, Phineas, Leper, and even Quackenbush. And if we can't trust Gene's words, then his explicit descriptions are the best way to understand Gene himself.
Finny's good looks are one of the basic elements of his persona in A Separate Peace. His physicality embodies his athleticism; the "cool blue-green fire" of his eyes reflects a certain vitality; even his weight – ten pounds more than Gene – reminds us that he is superior to his friend (or at least that Gene thinks as much).