Character improvement is something important, it is to insure that a character can be understood and not come off as a Mary Sue/Gary Stu.
When you have an idea for a character or have already created one, have the idea of what you wanted the character to be when you first thought them up, think of how well you know your character, you are the creator and you know them best or should
know them best.
Use a personality quiz to further develop your character, doing something like this will give you a better grasp of what your character is like and gives you more for a personality than say, "is generous, lovable, nervous". Taking a test for your character will give you more of a base to work with, meaning that you will have a basic understanding of your character. Are they introverted or extroverted? Are they sensitive or insensitive? Are they more likely to give ways to deal with a situation or give emotional support? Personality Test
The next thing you could do is take that personality result and make it into a better backstory for your character, take traits from the personality you got from the test and give explanations as to why they are that way. Why are they sensitive? Why are they selfless? Things like that.
You could also take this moment to open a Mary Sue/Gary Stu quiz, this will tell you if your character shows any Mary Sue/Gary Stu traits (e.g. tragic backstory, overly happy, shipped with a cannon character, liked by everyone, etc.) Don't give them a theme song unless it is completely original and made by you. Mary Sue Test
For CreepyPasta characters: Don't have them cry blood or black tears, the only time they cry blood is if they have a tumor, head injury, blood clot, tear in the tear duct or an infection, this shouldn't be used because it means that the character is injured or has poor health, not something you would want in a CreepyPasta. "Black tears" can only happen when someone is wearing mascara and begins crying, in rare, rare
, cases can a person cry black tears, once again, this has something to do with health and it usually bad. Don't make them nice, CreepyPastas are supposed to be creepy, private, people. Not outgoing, open-minded people. If they were making friends, then they would be caught quickly by the police or whatever. Never ever
make them bright, it gives them away. Real-Life Killers
For MLP characters: Don't make them related to Celestia and Luna or any of the princesses/princes, not only is this an extreme Mary Sue trait, but it also makes your character less memorable as this has been done time and time again. Don't make them more powerful than Twilight or any of the other princesses. Give them duller colors and not neon or eye bleeding colors. Good Palettes
For Sonic characters: Don't make them too bright or multicolored. Never
have them in a relationship with a cannon character like Sonic himself. Don't give them two tails, the ability to fly if they don't have wings, super strength, or super speed that is faster than sonic.
Tips for shipping: If you are going to make an OC x Cannon ship, make sure that isn't always making out or banging, partners take time away from each other because of work and things of the sorts. Not making a OC x Cannon ship would be much better for your reputation. And make sure that the relationship isn't always neat and clean, every relationship will have arguments. If it is a CreepyPasta, don't ship them at all, murderers rarely have time for dating. And if you are making an OC x Cannon ship in a fandom like Sonic, think about how many times that character has been shipped and what would be the outcome of another OC x Cannon.
Conclusion: Do research, make sure your character has strengths and weaknesses that don't go against their backstory or their personality. Make sure that they are pleasant to look at and don't have too many details or features. A simple looking character will be easier to draw in different angles and will be more accepted as a character. Your character doesn't have to be extravagant to stand out.
Have a good day!