Designing A Character
7 Tips to Get
You Started
Designing characters is something I absolutely love to do, even when I was a kid. But of course, designing a believable and interesting character can be a bit challenging. So, I'm going to give you a few tips and other things to keep in mind to start you off.
I'd like to add as well that you don't need to be an amazing artist or anything to design a character. There are plenty of references and templates out there for you to use and practice with (I know it's a cliché, but practice does make "perfect", or at least improvement).
And of course, take all this with a grain of salt. Art is pretty open, so in some cases, these tips wouldn't fit. However, I've found these things to hold up to be true and helpful.

References, tutorials, studies, and templates can all be found online, like on DeviantART or YouTube, to help you.
1. Don't Over-Do It
Keep designs relatively simple. You don't want to overstuff your characters with items. Especially if you'll be animating them! However, there are instances where a lot of items would be acceptable. If you have a character who collects a lot of things, it would work well. This tip is just something to keep in mind.
Take Lightningstar for example. Her color scheme is already very complex with all the blues and greens, so adding on all those items overcomplicates her design.

Too much busies your character and makes it harder for the viewer to take in the design and color schemes.
2. Saturation
You're free to use any color of the rainbow! That being said, try to balance out the levels of saturation and brightness of your colors.
Omaru's old and new creature forms are a good example of this. His old one had extremely bright and saturated colors, lots of them. The new version, however, dulled down some of those bright colors (all the while still having enough contrast to bring the viewer's eye to his face).

Keep in mind as well that colors add to the complexity. Too many will make the design hard to look at as well as more time-consuming to color for animation!
3. Poses, Clothes
When drawing a reference especially, pose the character and dress them with clothing that fits their personality. This way, a person can get an idea of their character without even reading anything about them.
Omaru's shy and reserved, so he wears clothes that cover him entirely and usually when I draw him, it's in a pose that reflects his hesitance. Drawing him in something, say, formal doesn't match him (specifically if it's outside a normal instance that one would wear formal attire).

I didn't do it here, but keep in mind the poses as well. Both of these poses match him well as a character, but a more "energetic" or outgoing pose would not.
4. Color a Personality
On top of that saturation balance, you want to use colors that make sense for a character's personality and background. This should work then with the poses and clothes to help further show their character without saying anything!
When I say this, I'm including ethnic backgrounds as well; However, feel free to break off and explore. Omaru's eyes, for example, are purple which is a very unusual color (even in their world). There's not really much significance to this choice either (really it was just purple was my second favorite color as a kid haha).

All of these characters I made conscious decisions when deciding on their colors (both their physical appearance and their clothes). Of course, not all of the colors had a heavy reasoning behind them.
5. Add Diversity
By that, I simply mean don't leave all of your characters looking the same. Add scars, freckles, moles, birth marks, and other such things to keep those characters individual.
You can also use color to differentiate of course, but these little details help push that even further. It also aids, once again, to discerning their character without saying anything.

Some characters won't need face details, as well. Take Lightningstar as an example; her color scheme is busy enough to keep her face plain (along with the character thing again).
6. Mixing Colors
Remember when deciding on colors that some colors really clash. Some distinctions standing out can be good, but too many can get confusing to look at (or just plain hurt your eyes, which you don't want).
This is sort of a reiteration of number two, but I'm more focusing on complementary colors for this. So, like purple/yellow, blue/orange, red/green... that sort of thing.

Now, you don't need to completely avoid putting two clashing colors together, but try to avoid having the entire scheme clashing colors.
7. Keep the Character in Mind
Make sure the design reflects how the character lives. If they're unathletic and eat a lot, then draw them a bit chunkier. If they're out in the sun a lot, then give them tanner skin.
And of course, this is just "The Golden Rule". I've said it a lot throughout this post, but it is really important. You don't have to completely match the character all the time or in such depth, but having a bit of their personality show through their design is beneficial for viewers to get an impression of them.

Also keep heights in mind. Séraphine is pretty short, for example, so compared to his environment or other characters, he's not going to be as high up, for a lack of better words.