@Bufuman
"Character development doesn't have to be blatant and in-your-face to be meaningful."
I can agree with this statement... to an extent. Not all character development has to be the obvious. However, I am taking the position that what has been offered for Itsuki is much too passive, and is based on fans of her believing her friendship with Fuutaro is organic and thus justified. I do admit, I am in ways ignoring parts of her development, but I am also calling into question how much background "development" is permissible. I again brought up Yotsuba because people denied very much that any development that pointed fingers to her actually having feelings were null; I've been in that position. However, I am saying that Itsuki's case of hidden context far surpasses Yotsuba's in the sense of it being much more speculative, or even shallow in the sense that all her development is hinged on an if together with the author only recently putting her in the race after many chapter showing otherwise. In ways, it could be said you are covering your eyes, believing more in a theory than the true value of things that are being laid out; princecup, if not you too, point fingers at other people for ignorance than deliberating how strong the points may really be. And if I am incorrect and your judgement is correct, stick to it and keep telling me off with evidence, not "you are ignoring XX."
"The way their relationship develops over time is by far one of the strongest points of the manga [...]"
I acknowledge, not too many romcoms, especially those that reach as much fame as Gotoubun has received, has a major heroine who has this much theoritical development, together with a relationship that goes almost platonic. However, I'd like to point out that I don't think this is the first ever "antagonist to acquaintance to friends" as I feel that I've observed a number of characters/ heroines going through a similar cycle (although in a different manner at times). That is just one of the ways authors will move a relationship forward, and it is a natural way people can develop. However, I don't think it can exactly be called natural unless you specifically wear the glasses to see it that way, especially with how subtle the developments are; tbf, there is no "normal" relationship development as anyone's romantic relationship can go in all kinds of ways. Heck, I do feel you generalize the other quints, because I don't think they go through developments "rapidly"; some can feel rapid (especially with how the series is paced), but those such as Miku and Yotsuba had a steady climb while Nino and Ichika have the more rocky ones. If you are referring to how romantically charged focus chapters are... well, certainly, yea, they are rapid in development, but a story is written to highlight these kind of key moments that push a relationship forward. But how about Itsuki until the recent chapters?
Essentially, I'd like to argue that Itsuki's relation to Fuutaro, while better off than a Tsundere mess, is not what I would call one of the strongest points of the series. It can be up there, but in this moment, I perceive it as a personal bias on your behalf than one of objectivity.
"[...] to disregard her own development as nonexistent just shows you aren't paying attention to her or Negi's story."
I've made it clear, I do not believe her development to be "nonexistent." No words in my mouth please.
Instead, I am suggesting a few things: Itsuki theoretically has development, but that is more to the reader's disgression until Negi goes forward with grounding more when Itsuki's relation turned from friends into the sudden romantic; I also want to point out how the surprise of Itsuki actually having possible romantic feelings for Fuutaro (thus being in the race) may not have unfolded in the best of ways as it kind of just appeared in one of the recent chapter (and no "you couldn't tell it was coming" because if you did read the chapters over and over, Itsuki's development around Yotsuba's reveal of being the actual Rena was sudden). I will believe you for saying that you believe each sister has a chance, but I am calling out to Prince of Cups above who chose their words specifically to scoff that Itsuki had the lead.
Also, I make the point of correcting Prince of Cups' statement of "show, not tell" specifically because I personally feel that Itsuki's development feels like a case of too little show AND tell. In that Itsuki's theoretical development until now was more subtextual content (if what context we can derive from their friendly relationship does lead to romance), that the notion of her entering the race was a shove down people's throat. More ambiguous subtext for people to play with, but not really directed subtext which we can make proper conclusions off of. Not everyone may pick up on minor details, but sometimes, you can read subtext wrong as a reader (hence why speculation fuel and theory crafting are so much fun for some).