Amir's first word was Baba, and Hassen's was Amir's.
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It seems as though Amir's first word indicates his affinity for and responsibility towards his father. This culminates in his concern for not fulfilling his father's understanding of what a "man" is. In Hassan's case, it seems clear that he is also denoting his responsibility--only in his case, it is responsibility towards a friend.
Amir explains that his first word was 'Baba' and Hassan's 'Amir', suggesting that Amir looks up most to Baba, while Hassan looks up to Amir.