In The Absolutely True Diary of a Part-time Indian, who contributes to Junior's development the most and how?

In The Absolutely True Diary of a Part-time Indian, Junior encounters several influential people including his parents, his grandmother, and his good friends Rowdy and Penelope.  However, arguably the most influential character in Junior's development is his former math teacher Mr. P.  When Junior realizes that he has been issued an outdated textbook, he throws the book at Mr. P in anger and expects that Mr. P will likely never talk to him again.  However, Mr. P invites Junior to have a conversation about education on the reservation.  During their discussion, Mr. P reveals that he has been complicit in a scheme to assimilate the Native Americans and that he has been a part of the dilemma on the reservation in which people give up hope.  Mr. P tells Junior that the only way to save himself is to go off the reservation for a better education that will lead to opportunities in life.  Once Mr. P suggests this plan of action, Junior seriously considers the proposition and eventually does end up going to Reardan.  Changing schools is a crucial part of Junior's character development as the change challenges him to both deal with being labeled a traitor by his own people and deal with the racism in an all-white community.