Is Your OC a Mary Sue?

The Basics of a Good OC

I once went through a phase where my OCs were all Mary Sues, and I'm here to teach you how to stop that from happening.

Appearance

Most Mary Sues have the 'stereotype' of having brightly coloured hair, unusual eye colours and so on. Unless the OC has dyed their hair or it is normal for their species, they should not have brightly coloured hair. If your OC is of a specific ethnicity, it is a good idea to look up the most common hair/eye colours of that ethnicity in order to make your character realistic.

Whilst two different coloured eyes IS a real condition (heterochromia), it is rare and unlikely to occur in the real world. If your character does have two different coloured eyes, make sure they are natural colours and not unusual ones, unless it is normal for their species. You can, however, have a character who's eyes have "mixed" colours (eg. green eyes with a faint brown ring around the pupil), as this is more common, but this should not be overused and should also consist of natural colours.

Personality

A good, solid personality is usually the more difficult part when creating a good OC. You have to make sure that none of the traits clash (eg. a character who is "kind around their friends, shy to strangers" can't be "murderous around their enemies and hot-tempered") and that their personality matches with their backstory. A character who grew up in a cold, unforgiving environment should be more easily scared, quieter than other people etc. and not absolutely fearless and obnoxious. The only excuse for having an odd personality is that the OC has some kind of a personality disorder. Again, this should not be overused, as it already is in a few cases.

A good trick and easier way to give your OC a realistic personality is by taking a test on the "16Personalities" website as if you were the character yourself. The results give you a detailed description of the person's personality and way of thinking, which should help you out if you are ever stuck when creating a personality for your OC. It also asks some questions which you may have never thought about for your character.

Backstory

Oh lord, the backstory. Most backstories which I have read say something like:

"Their loving parents died when they were very young from a terrible murder and now they are always sad and don't know how to love and no one understands them even though the OC reference picture at the beginning of the story was an image of Taylor Swift but then they fall in love and their partner always looks out for them and cares for them but the main character still feels insecure and questions their love even though their partner buys them like 50 flower bouquets every morning and showers them with kisses and tells them they look beautiful etc. etc."

Now while there isn't necessarily anything too weird about this backstory, it is very overused and overall just really boring. It feels like you're reading the exact same story but with different characters. Also, most backstories in fanfictions give the OC a very tragic childhood like the one above, and most of the time it's to give the story a plot, which I understand, but don't be afraid to write a story where the OC has a happy childhood. Every single fanfic that I have read where the character's parents are still alive has been a really good story with a plot just as good as the others.

Also, remember that your story does not need to have canon characters in them! Your story could be set in the Harry Potter universe, but minus the main canon cast. You see where I'm getting? Feel free to experiment with your ideas!

Relationships 

Most stories I read, the OC is in love with the cutest character in the fandom and hates the obvious evil character. This is quite generic, and while it can work in some ways, it is much like a dish served without any spices or flavouring. It looks good, but when you delve into it there is a feeling that it's missing something. Spice it up a bit! Maybe your OC works for the evil side instead? Maybe they're in love with some of the lesser-known characters?

Also, the OC usually is with a character that is already in a canon relationship with another canon character. This has multiple problems. First of all, when an OC has to break up a canon pairing to be with a character, they're usually nothing more than a replacement. I've read stories where an OC is dating someone, and they literally say and do the same things as the other canon character originally does in the movie/book. This is both unoriginal and boring, as it's literally the original thing with an OC in it. That's it. Wouldn't you think that because there's an OC instead of the canon character, the change of personality would cause them to say and do different things? 

Second of all, breaking up a canon relationship will seriously affect the main plot of the story. For example, if Edward from Twilight was dating another vampire instead of Bella, there wouldn't be any need for the Volturi to get involved and there would be no dilemma. See what I mean? Some canon relationships still need to happen in order for the original story to be the same. My suggestion is for such fanfics to be based away from the main plot or in an AU of some sort.