A problem I find a lot with people's characters is that their setting is unclear. One character is wearing a nirvana t-shirt while the other is wearing a medieval tunic, unless that is the setting then it doesn't make sense. Something that I often use while making characters is where and when they're from, as well as weather and religion there. If your character is deeply religious and live somewhere that is the same why would they be scantily clad? Maybe your character is from the Victorian times, then don't make them wear skinny jeans and a crop top. If someone is in a fantasy setting then why would they dress like Baby Gronk (unless that's the whole point then go off I guess).
Another problem (that technically is setting as well) is armor or uniforms that have all the men in actual things and all the women wear bikinis.
Armor for example isn't usually a decoration (asthetical armor exists but isn't meant for battle, unless they are elves, that's their whole thing). It's usually protective for battles that mostly had blades (when guns were introduced they were easily shot through) that went through vital organs (primarily in the torso and head). So characters with armor that only protected places that wouldn't be lewd, they would simply die.
Another problem with sexism-armor is armor that is shaped to the chest (like the Greeks often had even though they had their reason) would direct blows that were usually deflected by the shape of most armor were directed right into the heart. Not fun.
It's okay if everyone has it though, like Conan the Barbarian or Red Sonja.
2- Traits
Another thing not many people really think about is the character's traits, like goofy or opulent you would have to reflect that in their design. Someone who's bold and angry would be no nerd bookworm nor studious, they would probably be a punk or just look like that, which would be great comedy, and should be reflected in their design, otherwise the characters start to blend together into one boring and repetitive mass, and myorph into The Shifting Mound.
3 - Backstory
Who was your character before the story, who were their parents, do they have siblings, were they rich? If so, how would that affect them, here are a few examples:
Something a relative may have given them
A symbol of their House
If the're a noble they're wearing current fashion while if they're a peasant they might only wear sunbleached and torn cloth because they cant afford leather
They trained to be a wizard and this is their chance to prove themself, so they're still in their robes
A battle hardened knight would have armor with dents and scratches, any maybe they do themself
They were raised by wolves, so they would wear what they could find or kill
Maybe write notes on who they are, but this stuff is only useful really if there isn't a set uniform.
Even colors have meaning, and exploring that can make a character feel a certain way when you see them (See image).
But colors can mean certain things for different stories, Good Omens (2019) for example, with red, white, and yellow specifically; red as is in tarot cards represents femininity and hell while white is masculinity and heaven. We all know what yellow represents if you've watched the show and seen the theories (You'll need to after the ending).
If you're using clashing colors like red and green then one should be duller to improve contrast (If possible).
Shapes can help a lot to make characters more understandable, from traits to making a cast not be the exact same (although one may not have all of the listed), here are our shapes:
Athletic
Smart
Fast
Evil
Scary
Cute
Fun
Dynamic
Changing
Heroism
Strong
Stoic
Powerful
Impregnable
Powerful
Bye.