Through the course of the novel, the boys gain insights into their own personalities but they do not become, and cannot be, friends because honesty is missing from their relationship.

Both Gene and Finny grow as individuals as a result of Gene's reckless action. While the boys seem temporarily to grow closer after Finny falls, as Gene makes a massive effort at redeeming himself, the fact that he was responsible for Finny's injury renders him unable to be a true friend. Guilt comes to occupy a central place in his life, and he proves unable to get past the knowledge that he showed himself to be a shallow, vicious person.

For Finny, the fall and injury force him to reevaluate his view of life, but he ultimately returns to his core values. A gifted, winsome boy, he rarely engaged in self-reflection and took many things for granted, including his physical prowess. After Gene confesses to his evil action, Finny is angry at first and then refuses to believe him. Because Finny's subsequent decisions are apparently based in his innate confidence in human goodness, he seems a superior person to Gene.

The novel presents two basic obstacles to analyzing the boys' relationship, however. The first is that the novel is told in first person from Gene's perspective. Everything is his version of what happened. The second is that the events of their school days are far in the past. We are relying on the memories of a man who subsequently endured a war. We cannot know if his survivor guilt related to his wartime experience is being projected further into the past.

Throughout the novel, Gene and Finny's relationship develops and changes as Gene's insecurities indirectly cost Finny his life. Initially, Gene and Finny begin their summer session at Devon as best friends. Gene, who is fascinated and attracted to everything that Finny does, reluctantly follows his friend into some precarious, unforgettable situations. However, Gene's insecurities manifest into jealousy toward Finny as he begins to view Finny as more of a rival than friend. Gene drastically misinterprets Finny's motivations and feelings towards him by thinking,

I found a single sustaining thought. The thought was, You and Phineas are even already. You are even in enmity. You are both coldly driving ahead for yourselves alone. You did hate him for breaking that school swimming record, but so what? He hated you for getting an A in every course but one last term. You would have had an A in that one except for him. Except for him (Knowles, 24).

While Finny innocently trusts Gene and has no harmful feelings towards him, Gene deeply resents Finny for his natural charisma and effortless ability to succeed. Gene's insecurities and jealousy take a turn for the worse when he decides to purposely make Finny fall from a tree, which shatters Finny's leg.

After Finny becomes incapacitated from his injury, Gene's sense of remorse is evident. However, by making Finny his "equal," Gene's love for his friend is finally revealed in the mirror scene when Gene puts on Finny's clothes. Gene and Finny essentially become codependent on one another, and Gene's feelings toward his friend seem genuine. Unfortunately, Finny cannot deal with the reality that Gene purposely made him fall from the tree and dies of complications stemming from his initial accident. By the end of the novel, Gene feels at peace with himself and recognizes the true nature of his complex friendship with Finny. Gene's experiences with Finny have dramatically impacted his outlook on life, war, and his inner conscience as he comes to terms with his past actions.

The relationship between Gene and Finny changes and evolves, influenced by actions and consequences and filtered by changing perceptions. The changes are frequently by-products of Gene's insecurity and his constant self-evaluation.

At times, Gene and Finny are the best of friends, sharing adventures and feelings with complete openness and honesty. At other times, Gene considered Finny to be a rival and a detriment to Gene's ability to all that he could or hoped to accomplish at Devon.

you can't come to the shore with just anybody...at this teen-age period in life the proper person is your best pal...which is what you are...up like a detonation went the idea of any best friend, up went affection and partnership and sticking by someone

After Finny's fall, Gene's remorse consumes him and he eventually becomes Finny's surrogate, taking part in the activities that his action at the tree took away from Finny.

if I can't play sports, you're going to play them for me," and I lost part of myself to him then, and a soaring sense of freedom revealed that this must have been my purpose from the first: to become a part of Phineas.

Eventually, Gene makes peace with himself and what he had done to Finny, but he never forgets the lessons he learned or the impact Finny had on his life.

I was ready for the war, now that I no longer had any hatred to contribute to it....Phineas had absorbed it and taken it with him, and I was rid of it forever.