"It's you, pal," Finny said to me at last, "just you and me." He and I started back across the fields, preceding the others like two seigneurs.
We were the best of friends at that moment (1.38-9).
From the start, Gene's friendship with Finny isolates them from others.
What was I doing up here anyway? Why did I let Finny talk me into stupid things like this? Was he getting some kind of hold over me? (1.32).
The notion of equality is important to Gene when he considers his friendship with Finny. Much of his hesitation over jumping has less to do with a fear of dying than a fear of subordination, of blindly following Finny's desires.
I threw my hip against his, catching him by surprise, and he was instantly down, definitely pleased. This was why he liked me so much. When I jumped on top of him, my knees on his chest, he couldn't ask for anything better. We struggled in some equality for a while, and then when we were sure we were too late for dinner, we broke off (1.46).
Gene will later remark that few scenarios at Devon are not governed by rivalry. This, then, is how he conceives of his friendship with Finny. Wrestling together is a reflection of this healthy sense of competition, on which, as far as Gene knows, their friendship is based. This is why he feels so confused later, when he realizes Finny isn't concerned with competition between them.
He pressed his advantage because he saw that Mr. Prud'homme was pleased, won over in spite of himself. […] There might be a flow of simple, unregulated friendliness between them, and such flows were one of Finny's reasons for living (2.4).
If this is true, then Gene really did break something in Finny by betraying the trust of their friendship when he caused the accident.
It struck me then that I was injuring him again. It occurred to me that this could be an even deeper injury than what I had done before. I would have to back out of it, I would have to disown it (5.75).
There are two ways to interpret this passage. Either this is one of Gene's greatest moments of honesty (he would rather live with his shame than hurt Finny by revealing the truth), or it's yet another moment of justification (he pretends he doesn't want to hurt Finny in order to recant the truth and save himself from persecution).
"What I mean is, I love winter, and when you really love something, then it loves you back, in whatever way it has to love." I didn't think that this was true, […] but it was like every other thought and belief of Finny's: it should have been true. So I didn't argue (8.59).
Look at Gene's reaction to this notion: it should have been true. He's enticed by Finny because he's enticed by the world Finny has created – a world of youth and peace.
The moment was past. Phineas I know had been even more startled than I to discover this bitterness in himself. Neither of us ever mentioned it again, and neither of us ever forgot that it was there (8.108).
Phineas and Gene treat this outburst the same as they do Gene's earlier confession at Finny's house. Their friendship is predicated upon a suspended reality they build together. Or something else.
"Naturally I don't believe books and I don't believe teachers. […] but I do believe—it's important after all for me to believe you. Christ, I've got to believe you, at least. I know you better than anybody" (11.84).
Oh, irony. Or is it? DOES Finny, maybe, in some way, know Gene better than everyone else? After all, he does perhaps at some level know that Gene caused the accident and why…
None of them ever accused me of being responsible for what had happened to Phineas, either because they could not believe it or else because they could not understand it. I would have talked about that, but they would not, and I would not talk about Phineas in any other way (12.14).
This is Gene's moment of greatest loyalty to his friend. By treating Finny's death honestly, he reveres him in a way he never did when Finny was alive and Gene had to protect himself.