Palpatine, or at least a descendant of him. I just saw the movie, and it was pointed out to me how when she fights with the lightsaber she does lots of lunges and thrusts, like Palpatine did. Additionally, they made a big point of NOT saying her last name or clarifying who her parents are. If it was Luke, I feel like they would have just revealed that during this movie, rather than save it as some kind of predictable twist (and Luke hasn't hung out with a girl who wasn't his sister for the entire movie series now). Also if it was Luke, while it would keep up the Skywalker tradition of growing up on a desert planet, it does paint him as a terrible person for just leaving her there.
If she's related to Palpatine though...then things get a lot more interesting.
Also a friend asked me never to say "Reypatine" again, so I just gotta throw this out here.