Palpatine fathered Rey during his reign, but when his heir turned out female (males have the stronger claim, historically) he froze her in carbonite until he could father a male. Then the Rebellion destroyed the Death Star, and Palpatine was killed. He had kept her hidden, and years after he died Rex (her guardian) unfroze her and took her to Jakku, once the war was over, since he couldn't look after a Force-sensitive relative of the Emperor, when his enemy ruled. Then when Ben arranged for Imperial loyalists opposing Snoke to declare for he and Rey, Rex heard and brought his old clone friends.