How would you characterize the narrator's feeling for Mangan's sister? Why does he want to buy her a present at the bazaar?
The narrator feels nothing less than a complete childish infatuation with Mangan's sister. He views her with almost saint-like reverence, and indeed, the way she is described through the first person point of view makes her appear ethereal, almost angelic:
She was waiting for us, her figure defined by the light from the half-opened door... Her dress swung as she moved her body and the soft rope of her hair tossed from side to side.
The narrator's fascination with Mangan's sister borders on the obsessive - he waits for hours starring at the house trying to catch a glimpse of her and engineers situations where he can walk past her. He is clearly entranced by her even though he admits:
I had never spoken to her, except for a few casual words, and yet her name was like a summons to all my foolish blood.
It is because of this infatuation that he is willing to risk the anger of his uncle by going to Araby and buying something from the bazaar - he imagines himself as a knight embarking on a quest for a treasure that he can present to his true love. It is this childish, romantic notion that is dismissed by the epiphany at the end of the story.