I think that Junior does represent the idea of belief in self as critical for any hope of social or individual progress.  If one traces the arc of Junior's development, a common element is the belief in himself.  He struggles with cultural and ethnic identities of the good, as well as psychological battles that resound on a spiritual level.  In the end, this becomes something of extreme importance and the only tool of navigation for this is the idea of self worth.  At critical moments in the narrative, Junior ends up believing in himself and this what enables him to endure and eventually triumph.  The decision to leave "the Rez," enduring initial academic and social challenge at Reardon, his relationship with Rowdy, battling through basketball challenges, as well as making peace with the pain in his own life all reside in the realm of self affirmation.  If Junior does not believe in himself and does not possess a sense of self worth or dignity, he probably does not succeed in these situations.  Additionally, every challenge feeds off of the last one, which means that Junior is only able to persevere and triumph because of his growing sense of self confidence and affirmation in his own sense of self.  It is here where Alexie's work speaks rather loudly to the idea of belief in oneself as being critical.