Who is the main character in The Absolutely True Diary of a Part-Time Indian?

The narrator of The Absolutely True Diary of a Part-Time Indian is the main character. The narrator's name is Arnold Spirit, but he also goes by the nickname Junior. The reader finally discovers the narrator's name when he goes to his new school in Reardan, but before this he has been narrating in the first person without revealing his name.
The narrator introduces himself at his new school as Junior, but then he becomes flustered when the teacher calls out his full name, Arnold Spirit. In the text, it says

My full name is Arnold Spirit, Jr. But nobody calls me that. Everybody calls me Junior. Well, every other Indian calls me Junior. "My name is Junior," I said. "And my name is Arnold. It's Junior and Arnold. I'm both" (Alexie, 60).

Junior is an awkward and poor fourteen-year-old. He is thoughtful and quiet and likes to draw, read, and play basketball. He has several physical maladies, including "water on the brain" (Alexie, 1). Early in the book, Junior decides to leave the Spokane Reservation in favor of going to a better school off the reservation in the town of Reardan. The Absolutely True Diary of a Part-Time Indian is Junior's story about that transition.