Hello, I am SarahJuneBug, a hobbyist writer of fiction and enthusiast of art. I'm here to offer you tips on creating a truly original "original character," whether it be a fan character or your own series character. This will be divided up into sections bolded for ease of information.
What's in a Name?
Ah, yes. What to name your character. First things first, long names that are cliched/uncommon in the character's century/country/not even a word. Believable, preferably common names are more down-to-earth and relative to the audience. There are instances of exceptions to this rule (how many people have you known with a funky name?) but unless you have a reason for such (i.e. the parents just liked the name, etc.)
EXAMPLE OF AN OVERLY LONG, CLICHE NAME:
Ebony Dark'ness Dementia Raven Way
The problem with this name is, not only is it abnormally long with no explanation, it's annoying to read repetitively. There are also unnecessary apostraphes in part of the name.
EXAMPLE OF A MORE DOWN-TO-EARTH AND BELIEVABLE NAME:
Not eye-provoking. While you might question why you pick this name over the upper one (or you might not, as a seasoned author and sue slayer should already know) is that it doesn't scream sue the minute you read it and you're not pushed away from reading it.
Another big one. Looks don't have to be boring, as your character can have their own image; but there is a fine line between "having your own image" and just being ridiculous and overdone. Also, unusual colors in hair/eyes without a decent explanation (i.e. out of nowhere or with a poor reason, or a hair or eye color not provided by contacts or hair dye) is a bit annoying, even more so if characters "fall in love at first sight" because of these features, as well as the body shape.
With body shape, you can do a few things here. You can make them average, overweight, thin, curvy, anything you want. But to de-sueify, sometimes this may be a good place to work in. Use your common sense. Being "so thin they're anorexic but still hot" is not pleasing to read and vice versa.
Constantly saying how your character is "sooo hawt" or you going to great lengths to say how he/she is not attractive is off-putting. Again, use your common sense here.
You do not have to repeat what your character looks like every chapter; minor changes are of no need of being announced. I would reserve this for big changes or if another character is examining them, simplify the explanation as much as you can.
First things to know about personality are: make them so readers can relate to the characters. The more a reader is able to relate and connect to a character, the more they'll be able to have empathy for these characters and be able to really feel when something bad or good happens to them, so people will go "Ugh! I feel bad for them," when something bad happens, and when good "Oh, good for them!" When you can't relate, you can't have empathy.
Having common traits, even some off-putting ones that are honest, like "rude" or "mean" is believable. Also, "sexy" is not a personality trait. They may be "fiery" or "flirtatious," but "sexy" is to be avoided in your terms at all costs. Do not tell us who your character truly is. Show us.
Another issue with sues is they tend to be either overly happy or overly depressed at all times. Everyone has different emotions; it's how our brains work. Overly depressed characters can be sad or have hard lives, but it's annoying when they are sad all the time. Comic relief is a good way to solve this.
When a character is overly happy, it's frustrating and not possible to really relate to, for example, let's say their best friend just died. You would expect them to be sad, no? Well, what if they said "That's okay! YAY! I'm sooo happy and cheered up right this second!" It just doesn't sound believable and makes for a dull character.
This will be brief. Being a half of another race is usually acceptable for a character, provided they don't mention it every five seconds and has an original, interesting, and believable explanation. It doesn't have to be overly complicated, but it doesn't have to be overly simple, either.
Also, with common crosses like vampires or fairies or wolves, be careful. Either you can rock the mix or you can completely make it crash and burn. Even with a "perfect" species as you might call it, they always must have flaws. Once again, you do not need to announce these flaws; show us the flaws without saying. Seeing is believing.
Being a mix of three things is usually a character aiming for suedom, so be careful about your species mix.
A character's background should not be overly-magical or mystical at the same time. Make it believable with a good explanation. No one cares about a past with too much sorrow and doom, and no one cares about "I'm so magical because from my birth I was blah blah blah..."
Amnesia is a common thing we see. Opinions vary on this; some think it's cliched, some think it's okay, and some love it. Personally, I am fine with this provided not all of your characters suffer from it, and it makes it more interesting if they DON'T receive their memory back or only parts of it. But again, this is really a matter of personal opinion.
Being a prince or princess you can do, but you must be careful with it. Like a lot of things, this can be over-done. Everything in moderation. Rebellious Princess/Prince Syndrome (i.e. I DON'T WANT TO BE ROYAL I WANT TO BE NORMAL AND I'M RUNNING AWAY AND THEN SOMEONE WILL FIND OUT MY SECRET AND I'LL BE A BALL OF ANGST WAAAHHH...) is frustrating. We see it all the time. Maybe try having them like some aspects of it, and not some other aspects of it. They don't necessarily have to flaunt it, nor do they have to avoid the fact like it's the plague.
Pairing your OC with a canon character, provided the OC is not completely replacing canon love interest if one and not turning everyone OOC and making everyone, including the canon characters, fall madly in love with him/her is usually okay, including that they're not a diagnosed Mary Sue. CanonxOC pairings are a poor way to judge if a character is a Mary Sue.
Pairing your character with another OC is generally good; make sure both characters are compatible (you do the same for CanonxOC), not sues, and you're good to go.
Unless you're truly set on it, maybe your character doesn't have to have a love interest. But this is merely an option; remember that option exists, but you don't have to use it if you don't want to at all.
Pairing yourself with a canon character is iffy. There are so many opinions on this pairing that no matter what I say, it'd end in controversy. So above all, it's up to you to be the judge.
How do I know if it's a Sue?
Try showing your character when completed to a friend or an artist who will honestly give you constructive criticism. Most will be happy to tell you what is good, and what to be rid of. Don't take it as an insult; they are trying to help you. It takes practice to make a good character, so be patient. Put a lot of thought into your character before truly making it.
As a simple hobbyist writer, I am pleased to release my personal guide on creating a truly original OC. Education is the best way to writing truly great characters and fanfiction or stories. Keep trying, take advice, good luck, and make that original character!