There will be spoilers for both The Last Jedi, and The Last Watch
With The Last Jedi, Rey's profound abilities have caused many fans to refer to her as a "Mary Sue." I don't believe it's deserved, and the fact a previous example of her sort of character appeared in one of my favorite fantasy series has only given me greater appreciation of what Disney's doing with her.
The Watch is a Russian urban fantasy series by Sergei Lukyanenko. It follows two groups of supernatural beings that are at eternal war with one another: the Day Watch (good), and the Night Watch (bad). Both groups gain their abilities from an invisible force that is emitted by all living things called the Twilight (or the "Gloom" in the films). In fact, the less "Twilight force" you emit, the greater your ability to absorb and wield it.
In the fourth book in the series, The Last Watch, the Day Watch has grown too strong. Though unconscious like the Force, the Twilight creates a mirror-magician to bring about a new balance. The Mirror is a neutral character, and is only as strong as the imbalance. So when enough members of the Day Watch are killed in battles with the Mirror, it fades back into the Twilight, its mission accomplished.
While I'm pretty sure none of the writers of the new Star Wars films have read Lukyanenko's series, the correlation between Rey and the Mirror is pretty strong. Kylo Ren is powerful, and has decimated the ranks of the Jedi. Shortly after, the Force inside Rey awakens and she's every bit as good as Kylo. In fact, the stronger he seems to be, the stronger Rey becomes. Just like the Mirror, training isn't necessary as the Force gives her what she might otherwise be lacking. The balance must persist, and when it doesn't, the Force steps in to make it so.
The concept of a mirror-magician is foreign to the Star Wars universe, but I think it's interesting how closely it resembles what's happening in the new trilogy. It even fits into the gaps of knowledge we have about the Force since there hasn't been a time in recorded (canon) SW history where the adherents of the dark side and the light side have been so low, with no potential for balance. (Remember, Luke blocked his ability to access the force, which means there's no representative of the light side out there anymore). Call Rey a Mary Sue if you like, but I much prefer to think of her as something like a mirror. It may be new to Star Wars, but it's also an intriguing addition to the lore.
