Most of my characters formed from a random sketch or doodle, and a personality kinda just grew on them. Their fashion taste tends to grow with the character as I get to know them better (at least that's how it's been with my main comic, since it's my first serious comic).
Though as of late I try to think more critically, like: what's their role in the story? Would they want to stick out? Would they want to blend in? What's their monetary status (would they wear nicer brands? or thrifty? do they make their own clothes?) What's their feature (what's the first thing the average person notices about them? Height? Eyes? Hair? A big nose?)What's the setting of the world? (fantasy, sci-fi, modern day...etc) If I had to call them a trope, what trope would that be? (I find this makes it so easy cause you can go full blown tropey, and then peel away the layers you don't like)
I find character designing to be pleasant and relaxing, and these are the things I think of the most when I am being purposeful.
Though sometimes my entire goal is: I want them to look super cool. 