I'm talking about character design in general...
For example, here is two characters on the right in Azumanga Daioh I kept mixing up. Their designs are too similar. They did later on give one of them a tan which did help. Sometimes readers don't notice the small details that you put into your character.

I guess this is my own personal way of designing characters. What is the unique thing about your character that applies to only them that won't be just placed on everyone else?
Even from my other example, you can see a tall girl, a girl with glasses, a girl with big eyes and flat hair, a short girl with red hair, etc. That is what I mean by having a diversity in design, you could go to a more extreme case like Steven Universe but you don't have to.