Significant quotes:

I was beginning to see that Phineas could get away with anything. I couldn’t help envying him a little, which was perfectly normal. There was no harm in envying even your best friend a little” (18).
(This quote is important because it shows that right from the beginning Gene is jealous of Finny)

Finny saves Gene from falling out of the tree when he loses his balance (24).
(This show how much of a friend Finny is for helping Gene when he is about to fall)

“…Finny’s life was ruled by inspiration and anarchy, and so he prized a set of rules. His own, not those imposed on him by other people…” (26). (Finny is a person that will do what he wants when he wants)

Finny breaks the school record and won’t even share this fact with others (34-37).
(This shows that Finny cares about his friendship and didn’t want to rub it in their faces or make them even more jealous for the fact that he broke the record)

Finny tells Gene he is sorry for dragging him away from school and that Gene is his “best pal” (40).
(This shows that even though their friendship is falling apart Finny still tries to make things better between them)

“I should have told him then that he was my best friend also…But something held me back. Perhaps I was stopped by that level of feeling, deeper than thought, which contains the truth” (40).
(The truth is that Gene couldn't tell Finny he was his best friend because he was jealous of Finny)

“…up went the hope that there was anyone in this school – in this world – whom I could trust” (45).
(He thinks that he cant even his "best friend" and now says he cant trust anyone.)

“Finny had deliberately set out to wreck my studies….We were even after all, even in enmity. The deadly rivalry was on both sides after all” (45).
(He thinks that Finny is out to get him and to wreck his classes and academics.)

Gene purposely causes Finny to fall from the tree (52) and later admits it to Finny
(62). (Gene got his revenge on Finny but then feels bad for it and apologizes showing his maturity)

Gene is put on trial for knocking Finny out of the tree and is forced to face his wrongdoing (156-
169). (This just shows how serious the this jealousy has gotten between Finny and Gene)

Finny breaks his leg a second time by falling down the stairs (169).
(This puts guilt on Gene once again because he knows that Finny's life may be in danger and it is his fault)

You want to break something else in me! Is that why you’re here?” (176).
(Finny is now upset with Gene because he ruined their "friendship")

Gene is able to apologize (177).
(Gene again show progression in his maturity and is able to apologize to Finny)

Finny accepts his apology allowing Gene to achieve peace (183).
(Finny's shows his progression through the book here because he is now okay with Gene's actions)

Finny dies because of bone marrow escaping into his blood system and traveling to his heart
(185). (This puts guilt of Gene because he knows that if it weren't for him Finny would not have died and would have been able to live his destiny of going into the army)