Gene and Finny's friendship is defined by codependency, with each relying on the other to bring happiness to his life. At first, Gene and Finny seem to be normal teenagers. But Gene soon reveals himself to be a disturbed young man. He draws his energy from Finny, often remarking that he only competes in athletics and plays games in order to gain Finny's approval. In the first few chapters, Gene moves from praising Finny, practically gushing over his awards and achievements, to instantly suspecting Finny of sabotaging his grades and trying to ruin Gene's academic record. Yet Gene still needs Finny in a way, using him to bolster his own sense of popularity and acceptance.
After Finny returns to school following his plummet from the tree, the two boys' relationship becomes closer than ever. Gene still relies on Finny as his link to the rest of the boys; without Finny, Gene is quite unpopular and doesn't seem to have many friends. But the dynamic of the relationship has changed. Where once Finny seemed to be friends with Gene because he truly enjoyed his company, after his fall Finny grows despondent. He uses Gene to live out his own dreams, making him practice constantly and "train for the Olympics". Both of the boys know that Gene will never go, but there seems to be a vicarious nature to the relationship, with Finny living through Gene.
Through their relationship, both boys seek to find themselves, building their true identities. Finny knows himself throughout the book, and is comfortable in his own skin (at least at first). After his fall, he becomes more withdrawn and tends to hide his true feelings. It actually seems like he loses his identity as the novel progresses. The innocence and general good nature that defined him early on is lost in later chapters, as he continually deludes himself as to Gene's true intentions. Gene, on the other hand, hides his true identity from Finny and the others through most of the novel. Yet he reveals himself at several key points, such as his pushing Finny from the tree. The entire novel becomes Gene's recollection of building his identity, culminating in his return to Devon years later, where he is finally able to come to terms with what he's done.