Gene is a dynamic character because dynamic characters change over the course of the book.
Gene enters the story experiencing a flashback. Quite often, when a character is looking back, the purpose is to precisely demonstrate the change that took place in them. The storyline then moves him from a critically introspective and dilligent scholar to a much less than compassionate friend to Finny. Gene really struggles in the beginning of the book with Finny's attitude about breaking the rules. Finny cares little about being on time, attending class, and doing homework. All of these are necessary in Gene's world. They are moral issues.
This is interesting because throughout the book, much more serious moral issues don't seem to affect Gene's conscience enough to do the right thing about them. For example, he pushes his best friend out of a tree, allows it to be assumed that it was an accident, and then demonstrates less than care twice when Finny's injuries bind him.
Gene changes from moral to immoral. He changes from adoring and having great respect for a friend to having and showing very little respect for his friend.