@darkmatter923
[What you described are "character traits," not "character development." ]
My main point was she isn't "barely a character" as you implied in your first post. She may not get development as you would like it, but I am arguing she has meat to her character. She has a character (haughty and arrogant), history (outstanding achievement while growing up), desires (turning Yuuki in to a pet), flaws (superiority complex), quirks (loves dogs) and many more.
You also need to understand that development requires her to appear more often. Kyouka has a lot of development going on with her (particularly in regards to Yuuki) but she has plenty of screen time for that development to happen.
Ren however only appears for a handful of time, and much of her some of that is only for a few panels in a chapter. So i think it's ridiculous to demand major character development when she only appears for less than 20 chapters.
[Perhaps her realization of secretly wanting to be dominated is "character development," but beyond that, there's not much to her at all.]
I disagree with your notion that character development is only the changing of character traits. Development could also be adding and subtracting traits to a character, or adding a perspective of a character trait. Take Yachio for example, at first she's shown to look down at Himari(ch 11). She learns to respect Himari's abilities (ch 15, reinforced in ch 36). Then we learn Yachio actually cares deeply of her sister (ch 22). And that she's actually has been trying to protect Himari since they were little (ch 60 and 66). Yachio has gotten a lot of character traits added through out the series, but if we use your definition of character development, then she's only had one development which you would define as "barely a character". Which isn't true at all since character traits are being added to her over the course of the series, which makes her developed in my eyes.
The same goes for Ren. We are introduced to her in ch 46, She is self assured, cruel and haughty (ch 47), can act childish (ch 51), loves pets (ch 55), is an over achiever (ch 57), cannot stand being humiliated (ch 57 and 65.6), well connected and trusts her subordinates (ch 78), deeply cares for her subordinate... or is very controlling (ch 100). Note that there is no change in her character traits but a lot of character traits has been added to her. So Ren is a very fleshed out character in my eyes.
[That's the thing: 99 chapters in, I would have expected her to be humbled by at least somebody in some way]
She first made her debut in chapter 46, so technically she's around for 53 chapters, and she did get humbled by Yuuki in chapter 57. Perhaps you meant beaten in combat.
[ but not only has she not, no one has even come close to challenging her .]
Well, it's worth noting that Ren is the apex of the Mato defense force, if she's defeated shortly after she's introduced (say in 20 chapters) then the pacing would get out of whack. Remember during the 57 chapter after she's released, the series lore has been expanded. We are introduced to the other squads, we get development for ( a few) major characters, one of the 8 gods of thunder is defeated, and we are introduced to the other gods of thunder. If Ren was defeated shortly after Yuuki "humbled" her, then a lot of that development had to be cut because the author has to first introduce the new threat, Show the new threat defeated Ren, Yuuki has to gain a power spike to beat the new threat, and now the power creep has gone to ridiculous proportion in a span of a few chapters.
[It has me wondering: if it hasn't happened by how, will it ever? ]
The new arc introduced in chapter 100 does promise to contain Ren's "humbling" you want so much, so fingers cross for that to happen.
[a character is generally disliked for arrogance. ]
Ever heard of Rick and Morty? Rick is insufferably arrogant, but ...
[The character has to be really special for it to come off as likable and endearing, and that's not how I would describe Rin.]
He has other character traits that help him become endearing to the audience, particularly being smart and competent, he cares deeply for Morty, and he is deep in nihilism.
But what about Ren? Due to Ren not being a major character, she doesn't get as much screentime as Rick, however she has potential. Sure she may come off as unlikable, but not unredeemable. She hasn't done anything absolutely terrible. It is inevitable that her arrogance and recklessness will cost her. And I don't doubt that Yuuki will help her get back on her feet, hopefully endearing her to us in the process. I understand this isn't a solid argument. But give her time, the author has made several solid characters, and I wouldn't doubt Ren can become one too.
[I don't care if that kind of character is popular with some people, it's simply bad writing. ]
Conflating personal opinion as objective fact aside, I think you're wrong. She's not badly written, she has an appropriate amount of character traits for the amount of screen time she gets. And she has potential to become a well written character.