It's because she wants to know what your relationship is to you (ex: best friend, pal, good friend, girlfriend, one of the guys, crush). Otherwise, it could also be if you're already dating, "What do you think of me? Who am I in your eyes?" But, If she said it angrily (ex: "Who am I to you, a maid?!"), it means you probably mistreated her and you should apologize.
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