At the opening of the story, Finny is Gene's leader and inspiration. Finny is bold, daring, and capable of getting away with almost anything because of his deep and constant honesty.
Finny represents innocence and youth in this novel. Though he manages to maintain these characteristics, they come at a steep cost. When Finny dies, so does Gene's connection to the innocent world of boyhood.
Some of the costs of maintaining innocence, for Finny, are the loss of his constant honesty and the new trait of subterfuge. In the context of the story, these costs are attributable to Finny's broken leg caused by falling out of the tree.
After Gene causes Finny his crippling fall, Finny loses some of his innocence.
When Finny rationalizes the accident that caused him to fall, he engages in denial. Gene attempts to admit that he intentionally caused Finny to tall, but Finny rejects this admission. Later, Finny walks out of the trial to avoid facing the truth about what actually happened in the tree.
This avoidance is indicative of a loss of honesty. Another detail that points ot the same change in Finny is his fantastic lie about how there is no war. Whereas Finny once was able to get away with anything because he was no genuine and honest, after he breaks his leg he undertakes a strange and persistent lie.
Although he superficially denies the existence of World War II, he secretly goes to great lengths to enlist.
In the end, Finny cannot forgive Gene and this shows the greatest change in his character. Once he was carefree and open-hearted, but in the end he has been betrayed too many times to easily forgive.