It's no secret that Star Wars is a thematically rich franchise. With such great good guys and such bad bad guys, the series has often kept light and dark in very black and white terms. However, the series has also maintained a long tradition of focusing on the trials and tribulations of adolescence, and that coming-of-age storytelling will connect the moralities of Kylo Ren and Rey (no last name given). Their feelings will be more nuanced in The Last Jedi compared to Darth Vader and Luke. Director Rian Johnson explained:
Star Wars boils down to the transition from adolescence into adulthood. That's the heart of these films and Rey is most obviously the one that hangs on. But it's also Kylo. In the originals you project entirely onto Luke, while Vader is the scary other -- he's the minotaur. The fascinating thing about Kylo and Rey is that they're two sides of something. We can all relate to Kylo: to that anger of being in the turmoil of adolescence and figuring out who he's going to be as a man; dealing with anger and wanting to separate from his family. He's not Vader -- at least, he's not Vader yet-- and that's something I really wanted to get into.
By presenting audiences with a hero and villain who represent the same idea, The Last Jedi can adequately examine the good, the bad, and the ugly of adolescence. Per Rian Johnson's comments to Empire about writing Kylo Ren, you can take away the Force powers, the crossguard lightsaber, and the sincere homage to Darth Vader, and he's not that different from the franchise's new protagonist. Instead of giving the audience one youthful and innocent champion to relate to, the new trilogy has presented us with a similarly young and inexperienced villain who holds just as many relatable qualities as the movie's heroine Rey.
In a certain sense, it almost seems like the new trilogy of Star Wars films is combining arcs seen in the original trilogy and the prequel trilogy. With Rey, audiences get a morally righteous character who seems intent on doing the right thing at every turn -- not unlike Luke Skywalker. However, with Kylo Ren, audiences also get a tragically petulant character who feels far more prone to turn to the dark side -- echoing the eventual descent of Anakin Skywalker from The Phantom Menace through Revenge of the Sith.
With that said, clearly Rian Johnson feels their trajectories are tied together, and we will have to wait and see whether or not the "villain" and "hero" labels for these two characters fit down the line. Kylo Ren murdering his father in The Force Awakens doesn't seem as if it will sit well with him, and from what we've seen so far, Luke Skywalker seems wary of Rey when she shows up for her training. As the franchise tinkers and toys with the idea of "good" and "bad," (as well as how those ideas relate to emotional and physical maturity) it's entirely possible that the roles could shift in certain ways down the line.