A character’s design ends up falling apart if you lack some of these qualities, or they end up looking generic and/or unforgettable.
A character’s design should reflect their personality/purpose, or deflect it, if that’s their function. (Ex. gentle giants, scary monster w/ a heart of gold)
A character’s design should be consistent. When you consider all of the other aspects of your character’s design and function, are they all in agreement with each other? Does your design feel stable and believable?
When designing a character, it is particularly important artists consider balance. This goes hand in hand with considering shape when it comes to designing your character! Is your character tall, short, thin or round, square or triangular? It’s best to work from simple shapes, or else your character will not be as striking at first glance! When there’s too many things to look at, too many colours to acknowledge, our brains will become confused and block it out. This is why you see a lot of designs with very specific colour themes!
These are only a few examples of what you can do with just a few shapes! And you can exaggerate these as little or as much as you please depending on how you draw, because as long as the silhouette is clear and consistent, you’ve got a pretty balanced figure right there!
When drawing/designing humans/humanoid characters, it’s good to always keep in mind proper proportions and anatomy, but don’t get lost in it. Ultimately, that stuff is a guideline, and if you’re too focused on making your character have as realistic an “ideal” human body as possible, you’re going to get stuck with a lot of SAMEY DESIGNS!!!! Suddenly all of your characters are going to be 6 heads tall, lithe, with no real discernible silhouette!
Making your character unique comes from personal judgment. What makes a character unique? Their bodytype? Their hair? Their colour-scheme? Is there a specific asset to their body like spikes, detachable limbs, an extremely, inhumanly long neck? Is it their nose? Their eyes? Their clothing? ALL OF THESE THINGS CAN MAKE A CHARACTER UNIQUE! But you don’t have to go overboard with this! Remember, keep it simple!
As always, observe!!! Look at some characters in popular culture! What do you think makes them a good design, what do you think makes them stand out!?
Potential resources for you! Steven Universe Adventure Time Darkstalkers Mass Effect Wreck-It Ralph Psychonauts Bioshock Infinite Batman: The Animated Series Grim Fandango Monsters Inc./Monsters University
and even look at some live-action characters. They have specific looks and designs and costumes that are worth paying attention to!