So I've noticed a lot of things in the making of original characters.
To most fanfiction writers, original characters are known as 'OCs', and to others, 'MCs'.
And a lot, and I mean a lot, of original story writers have their OCs... In an unnatural manner.
They seem too perfect to be true; having extremely contradictory personality traits, having flawless looks, being über-powerful...
They just don't have flaws of any kind.
Etcetera. Makes you a bit sick in the stomach, eh?
Right. These OCs are known as 'Mary-Sues'. They aren't well-liked in the fanfiction world- I dunno about the original fiction world.
And I don't like them either. It outwardly shows that the OC creator hasn't given much research or effort into making the character.
Yeah, there aren't any set rules, but there are guidelines that most good authors use.
As a veteran story writer, I've asked well-reputed OC makers for advice. Here's what they generally say:
Make the character's personality and past make sense.
No one wants to read a story about a girl/boy whose personality keeps switching around. It makes the reader confused.
And give the character depth. Give them a PLAUSIBLE reason as to how and why they act that way. By plausible, I mean don't make your character's family all be murdered by a fire, and the aftermath is that your character is happy and fun.
Also, give them flaws. Not everyone is perfect.
Their pasts shouldn't be unrealistically tragic. I see this a lot in OCs in an attempt to 'de-Sueify' them. It doesn't. And if the past is that way, try and make the aftermath make sense.
And that's all! It's that easy.
And to make things easier, there's a plethora of Mary-Sue tests on the Internet that help. There are also character guides for every fandom (search a writing website like Wattpad, it may show up) all over the Internet.
Especially on Quizilla. There are a lot of helpful (and not-so-helpful) guides there. A user there that I learned from is TuttiFrutti999, who made a lot of hilarious and extensive guides on the making of plots and OCs.
And also, talk to the OC creators. They help lots! Believe me.
Anything else? Comments, questions, observations?
Google them- most likely, they'll come up!
Well, hope this helped with your (if you're an OC maker) OCs!
N O T E S;
 Unless, of course, your character has Multi Personality Disorder.
[ 2 ] She actually isn't on Quizilla anymore. Her guides are still there though, and if anyone wants to read them- they're here. They're a bit full of swearing, though. Not for the young.