To expand on this a little bit: Ryoko Kui takes the tools of character design seriously, and uses them with forethought and consideration to set her characters apart, give them personality and specificity, and thinks very carefully about what each piece of design communicates and how it interacts with all the other design in her story.
Body shape, face shape, noses, eyes, brows, hair, proportion, fashion, ears, posture, roundness and angularity, broadness and slenderness, posture... Kui clearly thinks about ALL of it, and incorporates all of it.
And this is part of what gives her story such a profound sense of taking place within a world, a whole world inhabited by thousands of people each of whom are as full and unique and distinct as every other one. You look at a group of her characters and none of them feel like Copy Pasted NPC Placeholder #3457, they each feel as though there is a life there, an individuality, even if they are never actually deeply explored in the story.
Compare and contrast with something like Genshin Impact's style of character design:
Now, I don't bring this up just to sh** on Genshin - its character design style is adapted very effectively to the kind of story and world it is trying to build, which is to say a gacha story where every part of a character is formulated towards the singular goal of appeal. It's a world inhabited by nothing but main characters, essentially, and it is a laser-focused power fantasy structured around constantly pursuing the high of maximum damage numbers pumped out by maximally cool and badass battle moves executed with maximal grace by physically perfect avatars who provide the player with maximal aesthetic pleasure.
But because of that, its character design style is under severe pressure to regress to the mean - i.e. skinny bodies, young bodies, beauty ideals, and a minimal amount of physical difference. This style of character design tends to focus all of its effort in colorful, detailed and attention-grabbing fashion and hair styles, and generally avoids "alienating" design features like, well, literally anything that could be conceptualized by anyone as "ugly." Big strong noses, for example, or larger ears, or wrinkles, scarring, skin folds and so on. Fatness functionally does not exist in Genshin Impact's character roster for this reason, and it's part of the reason why the franchise struggles so notably to design characters of color - the concept of "beauty" is deeply bound up in systemic biases of class, race, gender and nationalism, and since Genshin's character design ethos is "make every character as broadly beautiful as possible" it has to keep hitting the same limited set of beats over and over and over again, and it reinforces the biases it inherits with its inability to step outside of them.
So Genshin Impact characters have a tendency, for me at least, to all kinda blur together into a brightly colored cavalcade of lowest-common-denominator ambulatory clothing racks, characters whose bodies exist for the primary purpose of transporting a highly elaborate costume around.
Kui by contrast very very actively seeks out elements of physical difference, and incorporates them into her design process - she seems to delight in inventing as many nose shapes as possible, as many different kinds of eyes as she can think of, and the result is that she has a character roster which is recognizable even if you change or remove very important parts of their basic design.
Where Genshin Impact (and that style of character design) would severely struggle to make characters recognizable without their costumes, because the characters in large part are their costumes, Kui's design style makes characters extremely recognizable not only in and out of costume, but even if the fundamental nature of their bodies change across species, and it makes her characters of the same race and species eminently recognizable from one another, even while sharing many physical traits and aesthetic features.
anyway tl;dr Ryoko Kui is the best to ever do it.
