Phineas is the extreme example of all that's good in a character. He is straightforward and openly honest in all his enthusiastic, energetic encounters with everyone he meets.
He could also shine at many other things, with people for instance, the others in our dormitory, the faculty; in fact, if you stopped to think about it, Finny could shine with everyone, he attracted everyone he met.
Finny loves life and wants for everyone else to share his joy. He is constantly searching for the next adventure, pushing the envelope to test the boundaries and find out how far he can go in exploring new aspects of the world around him.
Gene, on the other hand, is not necessarily evil but is very insecure and concerned with preserving himself, which puts him on the defensive and easily causes suspicion in his relationship with Finny.
My brain exploded. He minded, despised the possibility that I might be the head of the school. There was a swift chain of explosions in my brain, one certainty after another blasted - up like a detonation went the idea of any best friend, up went affection and partnership and sticking by someone and relying on someone absolutely in the jungle of a boys' school, up went the hope that there was anyone in this school - in this world - whom I could trust.
The evil but unintentional result of Gene's perception of Finny at that moment in time is the retaliation against Finny's great natural athletic ability when Gene causes Finny to loose his balance on the tree limb.