How is Hassan heroic in the novel to Amir?

Hassan sticks up for Amir. He refuses to let the local bully take the kite. Hassan teaches Amir how to suffer with dignity. He teaches Amir to live with truth and the reality of life rather than fight against a losing battle.

I think Hassan's rape, in particular, at the hands of Assef shapes the rest of Amir's life. Issues of courage and cowardice, loyalty and betrayel shape Amir into the man he becomes.