Leper is a good minor character to look at as a person who reveals people's weaker sides. He is a nerdy kid who is rather eccentric, and so gets a lot of grief from the other guys. Any time he is around, the other characters turn mean, petty, and offensive. We see them belittle Leper, make fun of him, and go out of their way to make him feel ostracized. It doesn't paint a very flattering picture of most of the guys at the school. Leper, becuase he is a bit irritating and different, brings that out in them as he tries to make friends with them. Gene is one of his best friends, because at the beginning at least, he doesn't really participate in those behavoirs. However, later, when Gene goes to Leper's house after Leper's war experience, we see Gene in an unflattering light. Instead of being a supportive, helpful friend to Leper in his time of need and confusion, Gene instead displays a total lack of virtue as he insults Leper, yells at him, and runs away from the situation. Gene is selfish, catering to his own fears and insecurities over those of Leper's.
Brinker is another minor character who reveals people's flaws. He is buds with Gene and Finny, but in the end brings out their worst side. He forces Gene into a position in the boiler room where Gene lies and belittles another boy in order to take the focus off of him. At the mock trial, he drives Finny to cursing and yelling, something that is so rare for him that it is very shocking indeed. Brinker's rather aggressive pursual of the tree incident shows him to be a merciless and unkind person too.
Those are just a couple minor characters, but both of them serve to bring out some rather unpleasant traits in the friends that they have. I hope that those thoughts helped; good luck!