In "Araby," is the shop girl supposed to be the girl that he loves?
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By the shop girl you refer to I assume you mean the woman that the narrator meets at the end of the story at the bazaar. This is definitely not the woman he loves. Note how she is described:
Observing me the young lady came over and asked me did I wish to buy anything. The tone of her voice was not encouraging; she seemed to have spoken to me out of a sense of duty.
Thus we can see that this lady has no attachment to the narrator and the narrator has never met her before. The girl that the narrator is in love with is introduced at the beginning of the story, Mangan's sister, who lives on the same street as the narrator. Note the angelic, but also vague and shadowy way, she is described in the story:
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.
Note the way that the light almost gives her a halo, and the sparsity of the description indicates the insubstantial nature of her character and of the boy's romantic illusions concerning her. It is Mangan's sister with whom the boy thinks he is in love, and it is for her that he goes to the bazaar.