The changing dynamics of Gene and Finny's relationship and the revelations in Gene's personality are what move the plot along. Little does each of them know that in the end the two actually make each other stronger.
He is the other main character of the story. Gene made the choice not to enlist in the war, at the request of Brinker Hadley, when Phineas returned from the hospital and requested that he did not. The example essays in Kibin's library were written by real students for real classes.
Even more frustrating, Finny accepts his shorter than average height without difficulty, while the unconfident Gene tries to embellish his own physical stature by adding a half-inch.
As much as Gene enjoys these occasional thrills, he feels threatened — both academically and personally — by Finny's freedom. First, Gene's resentment of Finny is one of the main reasons why their relationship in the story does not last. This weight difference, "galling" to Gene, seems to prove that Finny stands as the larger, more substantial, somehow more generous, of the two.
One example of this is when Finny invents blitzball.