So, before I go into this, let me just define Mary Sue first. The definition of a Mary Sue is as follows- "(originally in fan fiction) a type of female character who is depicted as unrealistically lacking in flaws or weaknesses." (
https://www.lexico.com/en/definition/mary_sue). There is also another definition, there are multiple. This one says, "A Mary Sue is a character who is so perfect that he or she warps the world around them to display their perfection. It seems as though nothing in this universe except Mary Sue is even real, and is instead a plot device to make Mary Sue seem flawless and loved."(
https://www.urbandictionary.com/define.php?term=Mary%20Sue). By these definitions, Rey is indeed, a Mary Sue, but the meaning of Mary Sue goes further than just the definition, though I still do believe that Rey is needlessly overpowered without proper reasoning available to the greater mass.
As others here have stated, and we all know, Rey defeated Kylo for various reasons, part of it plot. I can see both sides of the arguments and believe that it makes sense and doesn't make sense that she beat him. Rey was indeed fresh and hadn't fought yet, so she was still full of energy. Kylo on the other hand, he had just killed his father, taken a blast from a wookie bowcaster and fought on, while also running through the forest and having another fight. Therein my problem is the fact that Finn managed to hold his own against Kylo, but that's another argument entirely. The problem for many is that Rey beat Kylo so easily and was able to use the Force just like that. It's plausible if you say that Kylo was extremely exhausted from his prior fights and trauma not so long ago, but still stretching it. However, I'll ignore that for now and say, sure, it's possible that Rey was strong enough in the Force to find her zen and be able to use it after someone told her she could. However implausible it may be, it's still a possibility that shouldn't be ignored completely.
Moving on though to another thing that makes people think Rey is a Mary Sue, her in general combat abilities without any training. Firstly, in the books and other sources, because of the mind link between Rey and Kylo, she is able to learn how to use the Force and fight with a lightsaber. I don't believe this is all that possible, or that she could get so good in such a short span, but let's say this did happen. The big problem here is that most people don't read the books, at least for Disney, but only watch the movies. In the movies,
no explanation is given as to why she's so good, she just is. Therein is the problem, she is so good and has no training, and no reason given within the movies, thus, a Mary Sue. This is a problem, as Rey is given no context for this, and Rey goes on to do things that took years for other characters to develop.
Next we come to general Force powers. In the movies, Rey had just learned that she is a Force user. Shortly thereafter she is able to mind control a guard, that is hyper indoctrinated to the FO cause, and should by no means be a 'weak minded' individual as the indoctrination would be a good barrier to overcome. However, Rey, with no experience using the Force and not having even been told, on screen at least, that Jedi can mind control people, does so after only the 2nd try on this trooper. This is very clearly being a Mary Sue in my opinion. At this time, Rey didn't have a mind link with Kylo, she had only just learned about her powers of the Force, and has zero training. Her use of the Force at this stage makes no sense, and her actually getting it to work even less. Then we come to her using the Force to pull the lightsaber to herself. First, let me point out that Luke did the same thing in the wampa cave, though he did have some small amount of training from Ben. So, this is implausible as Rey, again, has no real training in the Force, but can still do this. Sure, so she found her center in the Force as someone said, one problem there. This is assumed, in the movies we aren't told or even implied to that she found her center, no. She can simply do that and also fight off Kylo then, but I already addressed that. Anyways, the first instance is the most obvious point that Rey is a Mary Sue, according to the definitions.
Next I'll go into other characters, for example, Ezra, Luke, and Anakin, as they had been mentioned at some point. I'll just compare them quickly to Rey. So! Ezra, Luke, and Anakin are all powerful individuals in their own rights, some might call them Mary Sues, but I disagree completely on that point. All three characters develop over their time from a young and naïve individual into a powerful human that has earned their skill. Anakin goes from a kid on Tatooine to a beloved and very respected Jedi General over the course of the
entire war. He's a padawan, yes his taking down the control ship is Mary Sueism, but the character himself does develop and only has that moment, compared to Rey who is it the entire time. Anakin becomes a padawan, learns his trade over the Clone Wars under Obi-wan, has his fair share of losses and victories, loses some battles with main villains and wins some. But the key is, he lost his first battles because he wasn't anywhere close to being prepared. He lost his hand to Dooku because he wasn't ready to fight him. Rey just fought Kylo and won, no training. Anakin had some training, went against a more powerful opponent, and lost. Then he went and trained like crazy, become 10x better, and over several
years became good enough to win. His character developed from nothing into something. Rey is already 'something', there is nothing to truly develop as she already has everything.
If that's not enough, let's go into Luke. Similar/same story as Anakin. Starts off as a young boy from Tatooine who has no training, taken under the wing of Obi-wan, trains a bit and then goes through some losses and some wins. Ends up training, loses to Vader, trains some more over years and eventually good enough to win. Essentially the same as Anakin, though he came before Anakin technically. Finally, Ezra. Again, naive young kid from some backworld planet, not Taooine this time, lol. Gets hooked in with a group, trains under them, learns he has powers by accident in a fit of emotion and is then tought to use his new found powers. This takes years to develop his powers, and even then he goes through highs and lows, often coming close to falling to the Dark Side because he wants that power to win, to beat the enemies of the Rebellion. Again, he develops over many, many years and is seen training for long hours under Kanan to become better. He is trained by Maul as well, the holocrons, and needs to earn his powers. Rey just has them.
Seeing a trend there with those 3 characters? Yeah, so am I. So, to conclude, I do believe that Rey is a Mary Sue character. The reasons are simple, she has powers at her fingertips immediately, powers that took others years to train to get. She is also good with a lightsaber and other combat, despite never having picked one up to fight with before her fight with Kylo. Finally, her not losing anything so far and having no purpose at all. Sure, she could be going through an identity crisis, but if she has no reason to be anywhere, then her entire place in the movies is not that great, since she has no impact. In the OT, the characters fought to get rid of the tyranny of the Empire. Leia because of Alderaan and being raised that way, Luke because of his aunt and uncle, Obi-wan teaching him to fight, and having friends in the Rebellion. Han and Chewie fought because of money and finally agreeing with the ideals. In the new trilogy, Poe fights because it's his job and he supports the Resistance. Finn, idk why he fights, but again, different topic. But why does Rey fight? She
wanted to return to Jakku, she has no reason to do all these things, but she's dragged from one to the next. Sure, once she was involved, there was no escape, but her entire reason of becoming involved isn't that good(I can say more to this if anyone wants me to, but I think it's self explanatory). This is my own opinion on the entire subject, because I do believe Rey is a Mary Sue.
TL;DR
I agree that Rey is a Mary Sue, reasons included in the paragraphs above. Thanks for reading