You say this like its all a problem... but the story is written from Sora's perspective. We aren't told all of Kairi's motivations because they're not relevant to this story, although we are told more than enough to show how much she cares for Sora. She has a few conversations with Sora throughout the game, but for the most part giving her a more prominent role as a fighter and more dialogue wouldn't do a whole lot for this game although I would also have liked to see it. What is relevant is whether or not Kairi has the strength of will to back Sora up when he needs it and she does. That is all that we really need to see as a viewer to show how strong Kairi is.
Her character rises to every occasion presented to her even when she's at risk or isn't able to do a whole lot. That is both the strength and bravery of her character - I don't see why anyone would undermine these good qualities just because she wasn't a strong fighter in the game. In my opinion that is a disgusting way to think about her.
I'm sorry you think she was written bad, but she didn't need to be a particularly well written character in this story.
And of course her role in this story is proportional to how much Sora needs her. That was actually one of the main themes of this story wasn't it? How much Kairi means to Sora? She was a damsel in distress before she even went to the keyblade graveyard because Sora had already resolved to protecting her - he told her as much. And Kairi said the same to him. The fact that the story worked out the way it did was both because its Sora's story, hes the only one who couldve killed Xehanort, and because the story wanted to show how much Sora was willing to do to rescue Kairi in the end. It was all done to show the resolve of Soras character and his love for Kairi - if you go from that to saying this story is bad because it uses a trope almost as old as time itself to progress its story then you are the one at fault here. Your personal bias is not allowing you to see the value in this story.
The story is not written solely from Sora's perspective. There are many scenes which take place in which Sora is not even present (Riku & Mickey in the RoD, most of the Organization stuff, even scenes featuring Kairi herself in the Hyperbolic Forest with Axel), and many in which the lead within the narrative shifts between him and other characters.
For someone who, in bold type no less, proclaims that another person's bias is coloring their view of the story, you are very obviously leaning into your own bias, which according to your own commentary permits that it is acceptable to write a female character poorly, if it benefits the interests of a male protagonist. You defend this model of engagement using two lines of reasoning: 1) it is not essential to understand Kairi's motivations in order for the plot to work, and 2) this is OK because the "trope is as old as time itself."
Let me deal with point number 2 first: your appeal to tradition. There are many tropes and traditions within storytelling, and its accompanying methods, which are recognized to be intolerable in a modern context, both for reasons of heightened social awareness and because of aesthetic transitions out of certain cultural tastes and into others which causes some cliches to fall out of popular favor. The Minstrel Show was one of the most popular genres of live performance in the 19th and 20th Centuries in the United States, and while its enduring impact on the legacy of Black representation as iconography is still felt throughout the American media sphere, minstrelsy itself has become rightfully connoted with societal themes of racism, marginalization, appropriation, cultural and economic impoverishment, etc. and is no longer a popular form of entertainment. Silent movies, on the other hand, have become primarily novel relics of a time before dialogue and picture could be recorded and mixed in synchronicity and aside from statement features which specifically seek to assess the artistic value of silence as stylization, the cinematic standard is now sound films (in color, no less!). The transition from silence to sound affected the performances of actors (including which actors were considered viable) and the manner in which scenes should be shot, which ultimately affected the presentation of the material which, in turn, had an impact in which tools and tropes and even genres retained a popular function and which fell by the wayside-- we've seen a similar realignment over the recent advancements in computer graphics. We can apply this same strain of logic to
Kingdom Hearts 3 itself, which uses *gasp* updated graphics, disc capacity, and advancements in gaming hardware-- and even some awareness of the emerging tastes of its aging core audience-- to frame the story it wants to tell, which means that it does not behoove the game in any way to subscribe to "tropes as old as time itself" merely on the basis of their perceived longevity, anymore than it would definitely benefit the game to run on outdated hardware with lower graphical fidelity as a matter of maintaining tradition. Good authors and good game developers (like good characters) evolve with time, information and experience: bad ones metastasize. The bias inherent to your argument places it firmly in the latter camp, which is to say that kind of thinking is social cancer.
To point 1, since we have established that is not automatically beneficial or even acceptable to take a well-trodden path on the basis of mere precedence, then it should be reasonable to assume that it is not beneficial or acceptable to treat women as props and plot fodder for men in a narrative simply because that has been done in stories spanning back through humankind's historical memory. And here's where "the agenda" enters the dialogue, because the truth is, yes, I absolutely have a bias. I am biased in my belief that, basically, all characters appearing within a story should be written well for the sake of enhancing the positive qualities of the story (which stems from my biased belief that all stories should be well written), and also that within our present cultural context there is an additional incentive to write cultural minorities (women, PoC, queer folks, people with disabilities, etc.) with particular awareness and sensitivity as storytelling is ultimately a political act with social consequences, so it should be measured as a matter of public good and held to the high standards that entails. i.e. If someone writes a story with blatantly or subtly racist coding in overt or covert defense of those value systems which is intended for public consumption, they're engaging in a harmful act towards society and the quality of their work as it pertains to its social utility should then be measured on that basis.
So to your point that it doesn't matter if Kairi is well written because she serves her purpose, I have two replies:
First, if that claim can assume to be held true, then it doesn't matter if
anything in KH3 is well written so long as it serves its purpose. The reason to write a thing well is for the self-fulfilling fact of having done so, otherwise there is no incentive to write
at all and we might as well just exchange white sheets of construction paper with stick figures scribbled all over them and the words KINGDOM HEARTS 3 smeared with glitter glue along the margins and call it a day. It would be the same story, just told
more coherently with less finesse, but it serves its purpose so who cares, right?
And second, it
does matter that Kairi is well written because part of KH3's social mandate is to fulfill the interests and expectations of its audience (along with, yes, those of its author). And given that half of the franchise's audience is women and the rest is polysexual men and nonbinary witches, it can't fully succeed as a story without treating its girls like actual heroines and grounding them in a sense of real agency and autonomy-- it simply doesn't meet its mandate. It might be considered a "partial" success or people will lean into the ever applicable defense mechanism, YMMV, but it will never be considered a classic or a truly great story, just a middling, "divisive" piece of pop fanfic that 90% of its fanbase could have done better justice.
But it's OK, you're free to keep pretending I'm the only person in the world who's taken issue with this and that it's solely my failure to comprehend the deep value of this story, since it's clear you need a scapegoat.