Junior's character is not static, it is very much dynamic. He is constantly changing for the better, and as he gains experience, he gains maturity. From the beginning of the novel to the end, we see Junior lose some of his innocence, bitterness, and fear.
At the beginning of the story Junior is somewhat immature. He clearly hates his life on the reservation, but if there is one thing he hates more than the Indians who torment him, it is white people. He does not trust them and he is generally afraid of them. From the white dentist who doesn't give enough Novocaine, to the white teachers who are either "liberal, white, vegetarian do-gooders" or "conservative, white missionary saviors" (30). He only has one friend, and their friendship is probably largely rooted in the fact that no one else likes either one of them. He is very emotionally close to his family.
In just one year, Junior experiences several things that change him greatly. First and most important, he attends Reardan High, which is an all white school off the reservation. There, he meets kids like Penelope, Roger, and Gordy, who earn his trust through their kindness. He learns to stop hiding who he is, which is a very smart but also very poor Indian kid. He is a star on the Varsity basketball team, and must play against his former high school and best friend. In front of his entire reservation, he plays his heart out and wins for Reardan. All of these things lead to a temporary loss of his best friend Rowdy.
Other key events that take place over the course of the year are a sequence of surprising and tragic deaths that directly affect Junior. First, his grandmother is hit by a car and killed. Then, his father's best friend is shot and killed. Then, his sister dies in a fire. Junior experiences each of these losses, and mourns, and has a period where he is emotionally lost himself. But each of these result in him becoming even closer to the kids at his new school, and realizing that he has people other than his family who care about him. In all this he remarks about his own growth:
You probably think I've completely fallen in love with white people and that I don't see anything good in Indians.
Well, that's false. (152)
By the end of the novel, Junior is a more mature, more emotionally stable, and a more secure teenager. He reunites with his friend Rowdy. He has lost a lot of his fear of the unknown and looks forward to another school year at the all white school. And as he reflects on the year, he is aware of how he has changed.
I realized that I might be a lonely Indian boy, but I was not alone in my loneliness. (217)
He goes on to create an entire list of all the "tribes" he belongs to, which reveals that he has let go of many of his former grudges, fears, and prejudices, and is willing to make connections to people in other ways.