This is a major issue in our modern time, and extremely bad on Wattpad.
Mary Sues are an OP (Over Powered) character that's "perfect" and the world revolves around them and falls into disaster if Miss Sue is removed from it. They are also very common in FanFictions and will make the canon characters act out of character (Also known as OOC) and do things they'd never do in canon.
And for those of you who can't figure out what a Gary Stu is, it's just the male version of a Mary Sue. Pretty simple stuff.
Most Mary Sues are self-inserts where the author perfects themselves and writes them the way they wish they could be.
If you really want to know the difference between a normal character and a Mary Sue, then look at the photo I have provided. The girl on the left is a normal character of placed into a Neko story, she has a good bio that shows she's average but still has her individuality. The girl on the right is our beloved Mary Sue. See all the things she harbors? And look! Her father is Prince Vegeta and her mom is Queen Serenity! They're from two different shows and Vegeta only has a child with Bulma. This is a major flag for a Mary Sue. Know your canon people, especially when you do fan fiction crossovers.
Here are some tips on knowing not to make a Mary Sue:
1. Know the fandom/subject
Fan Fiction:
People, please don't make a FanFiction when you've only seen two or three episodes or have read only one chapter. Watch/read the entire series first, and if you're too lazy for that, then do some research. We have internet, don't we?
Original:
Make sure to research what you're basing your story on. Is your story revolved around the character being a doctor? Then do research on medicines and other common sense a doctor would need to know. Maybe you're writing a fantasy, that's where you can mess around a bit and create your own world boundaries, but don't make everyone OP.
2. Don't make them related to a canon character
FF:
It's okay for maybe characters with vague pasts, but characters like Harry Potter, Draco Malfoy, other major HP characters, Tris Prior, Katniss Everdeen, and others should not have a sibling that just randomly pops up out of nowhere when the canon character never mentioned them in the books or movies.
Now, if you're relating them to a character like Percy Jackson, Annabeth, etc, then it seems easier to believe that they are half-siblings since they share the same mythological parent. But do not make them a full sibling with sharing the same mortal parent! MS flag if so!
Original:
Now, your characters are the canons here, so the rule is more of a historical one if you're writing a historical book or maybe writing a mythological one. Make sure that you're aware this is the same type of rule. No making a character a descendant of Edgar Allan Poe, President George Washington, etc. (those who never had blood-related children), it's just not appropriate since they never had them. There's also the simple rule that you shouldn't make them related to a historical figure as well, nor should they be the parent to that figure either.
3. Don't make canon characters revolve and obsess over your OC.
FF:
Make your character a small part, they weren't in the movie/book were they? You need to make it seem like your character wasn't even existent in the series, sure, they could be that one kid in the group running away from the dragon or whatever happens in the story.
Example: Seth ran with the rest of his house out onto the grounds of Hogwarts, immediately shouting out a spell at a Death Eater.
This is a good example for a Harry Potter fan fiction because many students fought in the Battle of Hogwarts. And what if Seth was Slytherin? That's completely fine since a large amount of Slytherins actually supported Hogwarts instead of Voldemort and his minions. So don't be afraid to make them that if that's what you wish, just make sure to remember the movie or book scene of what first happened to the Slytherin house to make it go well with the canon. (I am rambling on about HP too much! I must stop xD)
Original:
The main character is not the center of attention. The simple way of putting it is to not make a Percy Jackson or Bella Swan. (I love Percy, don't get me wrong, but there were some major issues with him being the only one in the glory from what I saw).
4. Your OC is not always the hero.
FF:
Do note that there is already a set hero in the world you're writing for, Harry is the Chosen One (Did it again. Oops! XD), Frodo is the one who must destroy the Ring, Katniss is the Mockingjay, Naruto is the boy who is cursed with the Kyuubi in the Hidden Leaf Village, do I need any other examples? Remember why canon characters are there. There's a reason this is a fan fiction you're writing isn't it?
Original:
This depends on the exact role of your main character really. But do know they're not always the one saving the day, maybe one of their friends or a rival (on their side) accomplishes more than them at some point.
5. Give the OC a realistic name.
FF:
This is pretty self explanatory really. If you're writing an anime/manga fan fiction, most of them take place in Asia, so use names that relate to the region the anime/manga story. Some series that don't particularly match up to this would be Black Butler (In England), Hetalia (All the characters have names from their country), Attack on Titan (Mikasa is the only Asian in this series, last of her race sadly), Gunslinger Girl (In Europe, mostly Italy if I remember right). There's a lot of examples for you right there, so for anime fans, there's your major guideline to follow since I've seen it broken many times.
Also note series like LOTR require medieval names, and maybe a bit of research if your OC is a dwarf or elf. If it's a Star Wars OC, then look at the Online Star Wars Generator website, I've created some fun names for SW OCs with that. If you are doing Harry Potter (will probably use this the most since it's my expert FF field xD Sorry for using it so much) then do NOT use names that are mystic, fantasy, or something else that don't fit into 1980-90s UK.
Original:
Same application as above. It depends on the time and place of where your story takes place. Don't make it seem out of place unless the character was transported into a different world (An example being Colliding Elements. Not trying to advertise. Lol I don't know any real books that go into a different universe xD)
Another major note is that most characters have ONE middle name in western culture and no middle name in eastern culture. This is key to remember since most Mary Sues/Gary Stus have two or more middle names, but not all will, so don't think your OC is safe for following cultural guidelines.
Also don't use your name or a variation of your name for an OC, that's a scream to call your character a Mary Sue/Gary Stu.
6. Give your character ACTUAL FLAWS
Actual flaws are not things like being too pretty, super smart, wearing glasses, etc. THOSE DO NOT COUNT AS FLAWS!!!!!
You need true flaws to create a character, ones like a short temper, trust issues, loving their looks too much, they're very needy, etc. These flaws should make the character have real life consequences as well, if they're overly cautious, for example, then maybe they missed out on a cool underwater train ride because they were afraid that the water would destroy it? It's all up to you to make them more of a character.
7. Some things should be difficult for them!
There are those who may be prodigies at something, but that doesn't mean they succeed at everything!
Mary Sues/Gary Stus are good at everything, school, sports, combat, friendship building, etc. It's a headache with all that they're good at...
Your OC should have skills they struggle with! Maybe they aren't good with math, maybe they struggle with trying to improve at football (soccer for most Americans) or another sport, there's some things that they will struggle at! Give them challenges!
I'll give a simple example from my book Colliding Elements. The character Ash has troubles controlling her earth abilities, making her have to focus more on improving how to use them with her air powers to make a good attack. This is her struggle that only she alone can overcome. Your character should try to overcome something!
8. Avoid making yourself
Now all of us are guilty of this in some way, me included. We have based a character off of ourselves and we took the creativity out of the story/fanfiction we were writing by doing so. Try to avoid any personality traits that match between you and your character(s) and also try to avoid physical as well.
9. Have characters react differently towards your character
FF:
Think of how the canon characters act, would they disagree or agree with your character? Would one of the canons hate them (as in for their true personality, not jealousy >.>) or would another constantly pick on your character because of how jumpy they are? Take the canon personalities into account and don't mess their viewpoints up.
Original:
What characters get along? What makes the two best friends fight? What's the reason that one person is against the main character's idea? Each character you create has their own personality. Make an overall general personality for them and possibly act like them and find out how they would react to a situation. You need to tell if they may be offended by the main character's actions or not.
10. Don't screw up canon romance (FF only)
Simply stated, don't make your character end up with the main character or the guy they end up with or any other canon relationships.
Example canon couples include: Harry x Ginny, Ron x Hermione, Katniss x Peeta, Tobias x Tris, Naruto x Hinata, Sasuke x Sakura
Now if you were writing a ship fan fiction, then ignore this (I personally love Sai x Sakura tbh xD).
11. Take your time writing
Don't rush scenes, that's a major thing in Mary Sue/Gary Stu stories. Things happen just too quickly. Keep the story at a good pace and know that friendship building and getting to certain ranks/positions takes time. Always know you can write sequels as well.
That's about it on this tutorial really. I hoped it helped! If there's anything you have questions on or want help with a certain part on your character then comment below! (If it's something you don't want people to see just yet, then feel free to PM me instead).
And if you want to take a Mary Sue litmus test just to check where you're at with your character, then click on the external link. ^^