Try this: find a way to summarize the character in a single sentence, and base the visual design on that. From there, use the visual design to invent their background and personality. The two will eventually feed off of each other.
When I created Geri, the villain of Daddy’s Girl, I had a general idea of WHAT she was — a rich money launderer and matriarchal gang leader with a nasty streak — but not WHO she was. I tried to think of the most memorable design I could think of that involved “rich gangster lady.” I ended up with this:
Then I figured out the details of her background from there. She has cornrows and microbraids ‘cos they look good…but when did she start wearing that hairstyle? Did she always? She’s a white girl, so it’s sort of unusual for her. Maybe she rolled with a black gang when she was younger, and adopted the fashion sense of her fellow hoods’ girlfriends. So what did she look like before that? And how and why did she join a gang to begin with?
It sort of snowballs from there, and sometimes the character design changes as their background and personality evolves.