A hero is a person who displays courage or makes a sacrifice.
Initially, Amir doesn't appear to be a hero. He doesn't defend Hassan when he is being attacked by Assef and his thugs. And, he even convinces his father that Hassan is a thief, causing the boy to be sent away in shame.
However, over time, Amir comes to regret his decisions and and his inactions.
Amir becomes a hero when he returns Afghanistan and seeks to make amends with Hassan. Amir discovers that Hassan was killed, leaving a son named Sohrab. Facing great danger, Amir travels to Pakistan to get Sohrab. Ironically, Amir discovers that Sohrab is being held by Assef. Amir fights with Assef and takes the boy.
By finally facing Assef, Amir shows himself to be a hero.