For the purpose of the blog at hand, let's define a Mary Stu. A Mary (or Gary) Stu is commonly called an overpowered character. In the terms of creepypasta, or horror in general, it's a character so utterly overpowered, horrifying, and unescapable that it's ridiculous. It's that perfect villain that always wins.
Why are they bad? Because they're not exactly round characters. They fall flat, and no matter how well described you make the character's point of view of him/her/it, they're incredibly boring. Commonly, they're accompanied by poor description of the character itself other than "it was the most terrifying being X character had ever seen in his life!" Even when there is more than that, they're accompanied by a not-exactly-horrifying description.
Why is this? Because even if you set it up to where we sympathize with your characters and feel what they feel, it doesn't provide any form of feeling to simply say some creature was horrifying. It's also not realistic. Characters need to react realistically and logically to situations - they need to reflect how human nature works. I'm not going to be scared of a clown simply because you say he's horrifying. Unless he is threatening me and I'm (READ: logically) unable to escape, fight, or generally prevent the situation at hand, it's simply not a frightening aspect.
These characters oftentimes have no weaknesses or flaws, and frankly, they're irritating. Particularly, when they're the antagonist - it removes much of the conflict and excitement the story has and doesn't allow much room for build-up. How do you build a tense situation after your character supposedly killed the bad guy, if your bad guy wasn't even fazed and simply kept walking? Jeff the Killer, for example, is a Gary Stu. He can't die and has somehow amassed super-human qualities from getting burned with a non-flammable liquid. His description nor picture depict him as that much of a terror that he's made out to be. He's got no apparent weaknesses, and his character became literally immune to criticism with the exception of him being a Gary Stu.
SO, how does one avoid creating a Gary/Mary Stu?
- To start, give your character weaknesses and flaws. Unless they're some kind of god, there should be something that logically affects them in some way.
- Your protagonists should be able to slow them or even kill them in the right situation. A realistic character would probably be affected in some way by getting shot, having a tree fall on them, getting lit on fire, etc.
- They should be escapable in some way. This way could be incredibly small or a huge gaping hole, but there should be some way to get away from the bad guy.
- Tying back in with number 2, they shouldn't be immortal simply because they can be. For one, it's silly. For two, it's a red flag for a Gary Stu in general.
- Characters should react realistically to them. A fluffy kitten isn't the most terrifying thing in the world unless you give a reason as to why. Your characters should reflect this: if there's no reason to be afraid, they shouldn't be afraid.
- Tying back to number 5, that reason should be realistic as well.
- If they don't look like they can do it, they can't do it. What I mean here, for example: Jeff the Killer should not have the capability of infiltrating a police station and murdering everyone inside, and once he takes a bullet to the face, he should drop dead. Human character is human.
- Tying back to number 7, as well as number 1, 2, and 3, the character must have some form of limitation. An emphasized flaw, an emphasized weakness, or simply a character flaw can do this.
- Once forced into a situation, that character shouldn't simply be able to walk out because they can. This ties back in with number 3. If they fall off a cliff, they shouldn't be able to appear right behind a character 6 minutes later.
- And finally, put more development into the character itself. Don't simply refer to it as some horrifying thing. Put something behind that.
So there we go, ten ways to avoid Gary/Mary Stus. Enjoy. Anything else to add, feel free to do so.
EDIT: After a bit of mulling, it occured to me that sometimes a "Gary Stu" character may be acceptable in writing - to an extent. It largely depends upon character development and point of view. A common rule of thumb, for example, with children: Mommy is always a super-hero. This doesn't mean that the mother doesn't have any flaws; just none that the child can see. To some extent, it's acceptable and actually realistic for a dominant party to be considered "perfect" or "flawless" regardless of whether they actually are or not.