DON'T READ FURTHER UNLESS YOU HAVE SEEN THE THE FORCE AWAKENS/DON'T MIND SPOILERS. You have been warned...
So the game Disney Infinity has seemingly revealed the identity of Rey. In a battle with Kylo Ren one of his taunts is "Face me, cousin!" -which pretty much proves what most people guessed, that Rey is Luke's daughter.
https://www.youtube.com/watch?v=TNXRZd5vkdo Around the 20:31-20:35 mark of this video you'll hear him say it. So yeah, Rey is a Skywalker. I figured as much. Strong force senstive, good at piloting ships, etc etc.
Honestly though, we all knew it was his kid. C'mon. Just makes me wonder who Luke banged