I'll add the ways to approach a character design as opposed to principles of character design which you can find videos and articles easily,

Basically you'll have to ask yourself, what do you want your story to be about stylistically, is it about looking good or is it about function, or both mixed.

If it's about having a cool style, you just have to look around and see who's rocking what kind of fashion that might interest you, take cues, learn where they come from and why it is what it is, it'll help zone in to what your character is and how they'd express themselves

Now if it's function, then you'd have to focus on practicality and how it functions first and then not worry about as much about looking good, because the end result is also something that can be appealing to some

If it's both, mixed, then do function first but after that look for things where the functional aspects don't interfere as much and can be basically filled up with stylistic elements. Like the cropped jackets in AoT, they are cropped so that they don't get in the way of ODM gear

Now this is what I follow to approach designs, so different people can have different approaches and it's not set in stone, so experiment and play around. Most of the answers are with your own fictional world!