Finny can be read as a symbolic character. He represents the innocence of childhood. As the symbol for innocence, Finny is doomed to expire before the boys finish school and go off to war.
"Because of his ability to admit only as much of the ugliness of life as he could assimilate, Phineas was unique."
Of all the characters, Finny is the only one incapable of dealing with the war and with the truth of the accident in the tree. He prefers to ignore reality, in these instances. In this way, his innocence remains intact but is also clearly in peril.
Despite Gene's efforts, his grief and his guilt, his connection to Phineas is necessarily cut short. Gene must move on, grow up, and accept himself as a flawed person. The youthful ideals represented by Finny cannot survive intact into adulthood. They can only be carried as memories.
The story of this novel is not, ultimately, Finny's story, but Gene's. Knowles once stated that his novel presents "a story of growth through tragedy" and it is Gene who grows through a tragedy that he shares with his friend Finny.
...Gene will never again possess the innocence he recalls from the summer of 1942...
Finny dies and with him the innocence that once stood between Gene, life at Devon, and the future falls away.