We don't know much about Junior Varney, except that he steals Adam's bike and is about the same age as Adam. What he represents is the fears Adam has to overcome. Throughout the book, Adam is worried about someone stealing his bike, and the second he lets his guard down it happens. Of course this might just make him even more paranoid, but it also gives him an opportunity to see that it's not the end of the world. Bad stuff happens, our fears our realized, and we can still come out unscathed – and maybe a little stronger for it.