Basically everybody who saw Star Wars: The Force Awakens has the same basic theory about Rey's backstory. She's the child of a major character from the previous films.
But...whose? Well, we won't go into the big theories here, since I don't have the energy for a spoiler warning. But there's basically two major candidates.
...And now Mr. Sunday Movies suggests a third.
Star Wars: The Force Awakens, directed by J.J. Abrams from a screenplay by Lawrence Kasdan & Abrams, features a cast including actors John Boyega, Daisy Ridley, Adam Driver, Oscar Isaac, Andy Serkis, Academy Award winner Lupita Nyong’o, Gwendoline Christie, Crystal Clarke, Pip Andersen, Domhnall Gleeson, and Max von Sydow. They will join the original stars of the saga, Harrison Ford, Carrie Fisher, Mark Hamill, Anthony Daniels, Peter Mayhew, and Kenny Baker.