This debate is very interesting!
Just to throw another POV into the mix, when I design characters, where I start isn't very visual. I came to comics from novel writing, and so to me, the first step of the character design process is somewhat removed from any firm visual decisions. The first decisions I make that inform my characters come down to a mixture of role in the story/internal motivations vs external pressures.
For a main character, I focus a lot on what type of story I'm telling, why that person interests me, what makes them flawed and imperfect, and how they are living life in order to compensate for their flaws, but NOT fixing them. To me, the best stories usually attack the character right where they are their weakest, and the spend a lot of the first chunk of the story trying to solve the problem without engaging with their flaws, and then that moment of character arc/growth finally comes into play when the MC actually decides to change and grow.
For side characters, some of those aspects come in as well - the larger the role they play, the more likely they are to need their own narrative arc and the more likely they are to need a flaw that they're in denial about. But with side characters, they almost always play a "role" in the narrative of the MC, at least to some extent. So knowing that, what about them can be made to contrast with that prescribed role or bring them into conflict with the main character? Even with supportive friends/family, points of conflict are almost always more interesting than points of agreement.
When I do get to designing characters visually, I think I start somewhere with shape language/color theory personally, because those kind of strike the hardest and clearest fastest, then I build up details from there. Since my characters are in a realistic world and everyone changes clothes pretty often, I try to more come up with general palettes/rules for dressing my characters rather than iconic outfits. I might do it differently though, if I was writing something with superheroes or another similarly "costume" oriented piece.
In my most recent project, my main character was designed largely around circles. All her angles are very soft and even her eyes are more pronounced in a "circle" shape than all the other characters. Since I work in watercolor, there's also a particular shade of red paint I decided to only use for her hair and pretty much nothing else in the comic. All the other reds are cooler, so she stands out a little more.
In contrast, one of the other main characters is kind of all soft, sloping lines and another has much sharper features. It's fun!