In "The Kite Runner," if Hassan was the illegitimate son of Baba, then who was Ali?

In the novel, Hassan was the result of Sanaubar (Ali's wife) and Baba's adulterous relationship. Since Sanaubar was already Ali's wife at the time Hassan was born, all the parties involved simply continued with their lives as if the affair never happened. This was presumably done to protect Baba's position in his Pashtun/Sunni Muslim community. After all, Sanaubar was a Hazara (like her husband, Ali), and Hazaras were Shia Muslims; it would have been unthinkable...

In the novel, Hassan was the result of Sanaubar (Ali's wife) and Baba's adulterous relationship. Since Sanaubar was already Ali's wife at the time Hassan was born, all the parties involved simply continued with their lives as if the affair never happened. This was presumably done to protect Baba's position in his Pashtun/Sunni Muslim community. After all, Sanaubar was a Hazara (like her husband, Ali), and Hazaras were Shia Muslims; it would have been unthinkable for a man of Baba's position to admit to an affair with Sanaubar, a member of a hated sect.

After Sanaubar left Ali, he continued to raise Hassan as his own son. In the middle of the novel, we learn Hassan's true paternal heritage from the conversation between Rahim Khan and Amir. Rahim Khan confessed to Amir that Ali had always been sterile and that Hassan was fathered by Baba. Amir's reaction was explosive:

"You  bastards,"  I  muttered.  Stood  up.  "You  goddamn  bastards!"  I   screamed.  "All  of  you,  you  bunch  of  lying  goddamn bastards!"

"How  could  you  hide  this  from  me?  From  him?"  I  bellowed.

"Please  think,  Amir  jan.  It  was  a  shameful  situation. People  would talk.  All  that  a  man  had  back  then,  all  that  he  was,  was  his  honor,  his  name,  and  if  people  talked...  We  couldn't  tell  anyone,  surely  you  can  see  that."  He  reached  for  me,  but  I  shed  his  hand.  Headed  for  the door.

After he learned the truth about Hassan's parentage, Amir felt even more beholden to Hassan. He realized that both he and his father betrayed Hassan in their own ways. This terrible knowledge led Amir to fight for Sohrab's freedom as a form of redemption.