This is a great question, because Amir's development as a character is central to the book. In other words, only by knowing what Amir is like can we assess how he grows.
In the beginning of the book (chapter three, to be precise), Baba says these words to Rahim Khan about his son Amir.
"A boy who won’t stand up for himself becomes a man who can’t stand up to anything.”
By examining this quote, we can see something about Amir (and Baba). Amir is a boy without courage. He is weak, as he does not have the strength to fend for himself. The one who fends for him is Hassan; he is a fearless and loyal boy. When a few boys attack Hassan and rape him, Amir just watches and does nothing. Fear grips him and he is incapacitated.
The beauty of the novel is that Amir redeems himself by becoming a brave man. In the end, courage triumphs.
Let me give you one more quote that build on this theme. This time Amir speaks of himself:
“I actually aspired to cowardice, because the alternative, the real reason I was running, was that Assef was right: Nothing was free in this world. Maybe Hassan was the price I had to pay, the lamb I had to slay, to win Baba.”