Your question is very general. Gene shows many sides to his character through the course of the novel, with deepening awareness of himself and of the levels of his relationship with Finny as the story progresses.
Early on, Gene is frustrated with Finny, but also with himself for being unable to stand firm against Finny's ideas when he is uncomfortable with them.
I wouldn't have been on that damn limb except for him. I wouldn't have turned around, and so lost my balance, if he hadn't been there. I didn't need to feel any tremendous rush of gratitude toward Phineas...I never got inured to the jumping....but I always jumped. Otherwise I would have lost face with Phineas, and that would have been unthinkable.
Gene becomes aware that he is a better student than Finny but isn't sure how that affects their relationship and the fact that Finny is the better athlete.
He had won and been proud to win the Galbraith Football Trophy and the Contact Sport Award...If I was head of the class on Graduation Day and made a speech and won the Ne Plus UltraScholastic Achievement Citation, then we would both have come out on top, we would be even, that was all...My brain exploded. He minded, despised the possibility that I might be the head of the school.
After Finny's second fall, Gene has to deal again with the feelings below the surface of his friendship.
I didn't know what I was going to say at the Infirmary. I couldn't escape a confusing sense of having lived through all of this before - Phineas in the Infirmary, and myself responsible...We members of the Class of 1943 were moving very fast toward the war now, so fast that there were casualties even before we reached it, a mind was clouded and a leg was broken...My brief burst of animosity, lasting only a second, a part of a second, something which came before I could recognize it and was gone before I knew it had possessed me, what was that in the midst of this holocaust?
In the end, Gene is a very introspective character who struggles a great deal with his insecurities but does, with the hindsight of time, reach a mature understanding of himself and his experiences at Devon.