This is an interesting question, and the examples used to highlight it change at different points in the story. When the story first begins, it is more obvious that Gene's identity is more co-dependent on Finny than the other way around. They are best friends, and they love sharing that identity with the world:
"It's you, pal," Finny said to me at last, "just you and me." He and I started back across the fields, preceding the others like two seigneurs.
We were the best of friends at that moment.
While being best friends, I believe that Gene is more dependent on Finny because Finny is such a charismatic, care-free, athletic, fun, and confident kid that I believe that he would be just as happy with a different best bud. Gene on the other hand gains a bit of social status by being Finny's best friend. It's not the other way around. Finny remains who he is for a great deal of the story; however, he does become more dependent on Gene. In an understandable and sad way, Finny begins to...
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