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One of the biggest mysteries of the sequels is Rey's identity. Hopefully this secret will be revealed in The Last Jedi. if not, we will maybe get a clue or two. if the new trilogy follows the formula of the original (which it is so far doing) we will definitely get a surprise about her parentage right before the movie ends, during a climatic scene. In the mean time let's speculate and explore some of the theories going around the internet, and others that aren't.
UPDATE: One of the theories in this article was very close to what was showed in The Las Jedi. So be aware, spoilers ahead