I doubt Rey is Luke's daughter, that wouldnt make sense.
There has always been a balance in all the star wars. Since Kylo is Leia son it would make more sense that Rey is the Emperor's daughter.
Since Luke is dead, that means they need to bring balance back by killing the emperor since he never died apparently.
So who is left? Kylo who is suppose to be good but turn bad and Rey who is suppose to be bad turned good...Bringing balance.
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Think about one very important heirloom in the movies. Anakin's light saber. It's passed from Obi Wan to Luke, who loses it in the fight with Vader. He rebuilds his own, but then it somehow shows up in the Force Awakens and is literally calling to Rey. When she touches it, it shows her a ton of shit - about her family - Luke's family.
We'll attribute the saber finding its way from Bespin to Rey's hands to the Force because why not. Either way, it somehow found its way back to it's family and it's rightful inheritor - Rey. Passed from father to son to daughter.
Arguably, Rey is the main character of the new trilogy. The 7th movie is called
The Force Awakens, 8th is
The Last Jedi and the 9th is called
Rise of Skywalker. In Last Jedi, Rey says "something was inside me the whole time, and now it's awake." Shes talking about the Force... Awakening. This is alluding to the 7th title.
In TLJ, Luke says he won't be the last Jedi, in this case inferring that Rey will be the Last Jedi.
Also the last 8 movies have all been about one family - The Skywalkers. So taking all of this together, it clearly points to The Rise of Skywalker essentially being the Rise of Rey. Why? She's a Skywalker. More specifically Luke Skywalker's kid.
The OT movies are about Luke Skywalker. The Prequel Trilogy is about Anakin Skywalker. The Sequel Trilogy is about Rey. Rey Skywalker.
Also the one thing that is constant about Skywalkers is that they are absolutely proficient in the Force. They may be idiots, or turn to the Dark Side, but they are proficient Force users and attuned to it like motherfuckers. That's a hard core maguffin, but it is just a fact for the movie's universe. Rey just happens to be a strong force user right out the gate.
A friend made the argument that she could even be Leia's kid and Kylo's sister. I mean, sure, maybe?
But are you gonna tell me that a woman who for two movies has been pining and hoping that her lost idiot sheep of a son will return to the light, and who pushed a watermelon out her vagerj isn't gonna be like "I wonder where my daughter went..." Especially a Force Sensitive Skywalker? And bumping into her kid later in life, both being Force Sensitives, she's not gonna recognize her own kid? Not to mention Han wondering where his daughter ever went. No way. She's Luke's kid.
And Kylo Ren talking trash about her parents being trash from nowhere? You're gonna trust what some whiny daddy-issues Vader wanna be says?
As far as people saying it's Palpatine's kid, now THAT I dont see any evidence of at all. It it truly wouldn't make any sense. Maybe he makes her turn like he did Anakin and tried with Luke? But no way is she his kid or even Grandkid. I don't see anything that points to that being the case.