The first factor that contributed to shaping Junior's life was his environment. Growing up on the Spokane Indian Reservation, Junior is constantly teased, beaten and bullied because of his physical appearance and handicaps. This drives Junior to want to become a rich and famous artist and leave the reservation. After talking to his teacher Mr. P about the “hopelessness” of the reservation this is also what makes Junior decide to attend Rearden as well.
Another important factor that led to the development of Junior's character was his perceived "ugliness". Junior is obsessed with his outward appearance and the misfortune it has brought upon him. The entire first chapter is a list of his disabilities, unfortunate characteristics and the consequences of his circumstances. However, his social isolation on the reservation is what creates his dry humor and his love for academics.
Lastly, Junior's need for approval from those he seems to "hate" was the most important factor in developing his character. Despite his criticism of those on the reservation and at Rearden, the majority of this book is Junior attempting to find his place in the world and fit in. This is especially the case with Penelope where he believes he must change everything about himself to win her love.
Ultimately, Junior finds that like his father had said to him, the first day of school at Rearden he was a "brave warrior" because he made the "attempt" to improve his life. Junior is upset that he will find a better life in the "white world" but knows despite his loneliness he has made the right decisions.