Gene describes Finny as almost super-human. He's some combination of a Greek god, a mischievous devil, a super-athlete, and an earnest kid. There's practically a divine glow coming off his body, so much so that we start to wonder, could anyone like this really exist? Could Finny possibly be everything Gene says, or are these descriptions the product of an overactive imagination and a teenage mind just a wee bit prone to hyperbole?
But even if you keep in mind the old unreliable narrator deal, it's hard to not be completely taken in by the character of Phineas. He's charismatic, pure of heart, naturally skilled, and most importantly, vulnerable in an almost palpable way. Finny may be a hero, but he's far from invincible, and his ability to be broken keeps him human in our eyes, no matter how Gene would have us see him. Finny's vulnerability isn't just physical, and accordingly, he suffers more injuries than a mere broken leg. Check out that scene on the beach, when Finny nakedly declares that Gene is his best pal. He makes himself vulnerable emotionally, and by not responding, Gene takes advantage of that susceptibility.
Again and again, Gene hurts Finny in these non-physical ways. When he studies with vigor so as to beat Finny in the Devon game, he does so quietly, not wanting Finny to understand him as he understands Finny. He harbors a secret resentment and, in doing so, takes advantage of Finny's naiveté, his goodness, his trust. These qualities make Phineas who he is, but they also put him at risk. Just look at the things he says. "When you really love something, then it loves you back, in whatever way it has to love" (8.60). He can't possibly imagine his friend as having caused his accident, because he loves him too much. Finny suppresses his memory of that day and even disregards a full-blown confession from Gene, all in the name of this sort of mutual love.
Gene recognizes his friend's vulnerabilities, both in retrospect and as a sixteen-year-old. He says of his friend, "Phineas was a poor deceiver, having had no practice" (8.73). But the truth really comes out in that final conversation between the two boys, when Gene explains how useless Finny would be in the war. That's just it. Phineas is useless in a world of hostility and fighting. For all his skills, for all his god-like abilities, he's still rendered vulnerable, hurt, and even killed because of his character.
Finny may understand this to some degree, which goes a fair way in explaining his somewhat unorthodox way of living. "Phineas created an atmosphere," Gene says, "a way of sizing up the world with erratic and entirely personal reservations, letting its rocklike facts sift through and be accepted only a little at a time, only as much as he could assimilate without a sense of chaos and loss" (13.46). Indeed, Finny does "create" his own version of reality. He's got a system of rules to govern behavior ("never say you are five feet nine when you are five feet eight and a half"), an outlook that ignores every negative human trait, and a dependency on denial that's like nothing you've ever seen – remember, he's convinced himself that the war does not exist and that Gene will participate in the 1944 Olympics. Finny basically turns the world into one big playing field. And since sports are purely good and no one ever loses, Phineas is always in his element. When certain events threaten to pull him out of that fantasy world, Finny reacts by retreating farther into it – and by taking Gene with him. "He drew me increasingly away […]," says Gene, "into a world inhabited by just himself and me, where this no war at all, just Phineas and me alone" (9.17).
In this world of his, Finny tries to live through Gene. Gene is supposed to play sports for him, to "become a part" of him, to compete in the Olympics for him – to do all the things that Finny can no longer do. That's why Phineas is defensive about letting Brinker help him with the crutches, but is fine with taking assistance from Gene. Gene isn't just another guy, he's an extension of Phineas. It's not until Brinker's investigation in the Assembly Hall that Finny is really forced to face "the facts," as Brinker says, to come back from his world of fantasy and see Gene as he really is.