Please keep in mind that these are all just opinions, really. I am not telling you that you can't do these things! (Not that I have the authority to do that anyway). More than anything, these are just things to take into consideration when creating a character for a novel.
Heterochromia. This is the condition where one's eyes are two different colors. I just did some quick research on Google and according to what I found, this happens in somewhere between three to eleven out of one thousand people. So let's be optimistic and go with eleven. That means the chances of a person having heterochromia are about 1% at best. Despite these tremendous odds, deviantart is littered with OC's that have heterochromia. Can you have an OC with heterochromia? Go right ahead. Please be warned though: your character will scream Mary Sue. (Forgive me if I sound rude- this trait drives me absolutely batty.)
- Exceptions: In Cowboy Bebop, Spike has one brown eye and one red eye. It is later revealed that his red eye is fake. In Naruto, Kakashi has a red eye as well due to a transplant. In Graceling by Kristin Cashore, there is an entire race of people who possess heterochromatic eyes. They are called Gracelings, and they all have a "Grace", which is sort of like a magical talent. In all of these examples, there is a reason for the heterochromia: not "just because".
Crazy Colored Hair. Many of us love manga, anime, and shows where purple hair is nothing to think twice about. However, sometimes we forget that this isn't considered "normal" and throw it in our stories before we realize it. There's certainly nothing wrong with colored hair, just make sure it fits into your story. If your characters are a bunch of humans adventuring in the medieval times, they're probably not going to have turquoise hair. If your character is a modern day teen, it fits much better. However, what about school? Many schools have dress codes that prohibit colored hair. Whatever the case is, just make sure you think it through.
- Exceptions: The fun thing about this is that it's easy to create exceptions. For example, my character Tsuki was given blue hair by my oblivious thirteen-year-old anime-obsessed self. Seven years later, it occurred to me that there was no way they had hair dye in those times. However, Tsuki isn't human, which gave me room to play. I decided to apply the trait of blue and purple hair to his entire race, which actually led to a nice bit of story development. See? Not so bad.
Beautiful, long, flowing locks of hair. Please see this lovely guide to hair by MissLunaRose. Especially if your character does anything other than sit still and look pretty. fav.me/d3szjg0
Manga Bodies. Another form of conditioning that comes from reading too much manga (or watching too much anime) is the everyone-has-a-perfect-body-syndrome. When deciding your character's weight, height, and body type in general, there are far more things to take into consideration than attractiveness. For example, if you are writing in a medieval setting and your character a thin, lean female, she's probably not going to attract many men. Heavier women were preferred in those times. To quote the movie Dark Shadows, "She has the most fertile birthing hips I've ever set eyes upon." Just as it was believed that a fat baby was a healthy baby, women too were thought to be unhealthy if they were too thin. Aside from that, take into consideration the things that your character does. If they are an athlete, they will almost certainly have a different body type than if they were a gourmet chef. If he is a rich and lazy King, he will have a different body type than his knights. Also, don't forget that women can be tall and men can be short! It happens!
Character and Personality Traits
Age 9243, Appears 10 This is another one of those "do it if you have a reason, but not just to do it" things I think. I see a lot of characters that are immortal creatures such as vampires or souls in other bodies or original species that are crazy super duper old but appear very young. Immortality is a really fun thing to use in stories, but your character doesn't always have to look super young. Perhaps they were turned immortal when they were in their 30's or 40's, or maybe older! Or perhaps they can change their appearance- it certainly would come in handy to appear much older at times. The other end of this is the age. To be immortal, you don't need to be 9243 years old. Heck, you can "appear" ten and only be twenty! The age gap does not have to be mindblowingly enormous for your readers to get the point.
Brave Warrior, Cowardly Child, and that Bitchy Cheerleader Snob. If I had a penny for every time I opened up a book and found that the main character was a misfit highschool girl being tormented by Queen Cheerleader, I'd be able to buy... well... better books. I'm not saying it doesn't happen, because it does! I went through it! But let me tell you a story.
- Once upon a time there was a super popular girl in my volleyball class. She was the epitome of "preppy" and I steered way clear of her and her posse. She reeked of evil and I knew to stay away. Then one day, as I was leaving the locker room to find my equally misfitting friends, someone called out a very rude comment. Before I could even come up with a response, Queen Prep turned around and told them to "shut their fucking mouth and leave her the hell alone". She didn't have any reason in the world to do that - I'd never even spoken to her! But she risked her popularity points to defend some awkward goth kid, and that's what matters.
Sappy story aside, keep in mind your stereotypes. Your misfit main character may think the Cheerleader's only goal in life is to sabotage her in every way possible, but what if she only does it because it makes people laugh? What if that laughter is the only way she can make it through the day before she goes home and tends to her eating disorder or hides from her abusive parents? Everyone has problems. You can make your characters far more interesting by remembering that. Not all heroes are brave, not all thieves are liars, and not all evil masterminds are out to conquer the world.
The Wallflower. I think that all writers should take the time to write out a brief (or not) life story for their side characters. Why? Because I think a lot of side characters are created just to be there. To fill up space. Every person in this whole world has their own story, even if you don't know it. The girl that rings you up when you buy groceries? She has a family and a life and a favorite food. The guy you bumped into on the subway has a home and maybe a dog that he loves more than anything; he has hopes and dreams and fears. So do your side characters! I think this happens with sidekicks especially- the author creates them to compliment the main character, then forgets that they have a life outside of that. Make sure all of your characters have a reason! If they don't, write them a little side story! Even if you don't share their story with anyone in the whole world, knowing it yourself will help you to write them better.
This Writers Guide is still in progress. It is as complete as I can get it now, but I'm sure I (or you!) will think of something else to add. Please share if you do!