Writing on this topic, Finny is a character you will want to examine along with Gene.
Finny can be read as a symbolic character, at least in part, representing youth, childhood and innocence. The early sections of the novel identify Finny with a care-free attitude as he leads the boys in games and diversions. Later, the novel's conflicts deal with Finny's realization that the world is not as easily knowable nor as blameless as he initially seems to believe.
Finny's development in the latter half of the novel can be seen in terms of loss of innocence.
Gene thinks often about Finny's character and provides a good deal of direct evidence for an essay dealing with Finny as a symbol of innocence.
A number of specific instances force Finny to abandon some of his naivete and innocence, either by implication or by admission.
After Gene causes Finny his crippling fall, Finny loses some of his innocence.
Gene's narration - or the whole novel - can be described as a recollection of Gene's own loss of innocence. Only as an adult, does Gene finally come to terms with this loss, as it includes not only his own guilt but also Finny's death.