You know, that happens to be a question the series doesn't answer very well.
A video game series dreamed up and created by megalomaniacal megacorporation Square Enix that asks the questions 'What Popular Properties can we shove together to make more money?', KH is infamously a combination of Disney World and Midgar from FFVII, but the Final Fantasy has to bend to Disney World's standard, not the other way around. It has its own internal worlds, characters, and plotline, but it thrives on settings that hop from one incredibly distinct setting to the next, making its central characters adjust to each new world and its logic as they go.
I played these games when I was young, though perhaps not as young as you're thinking; my parents were anti-video game and KH was the first game they deemed safe enough to let me play. My sister and I would bring our popcorn and koolaid down to the basement, grab blankets, and stay up as late as we could stand playing together. We played KH1, Re:CoM, and KH2 right by each other on the couch, top to bottom, several times (and then moved on to playing Final Fantasy game together once the folks accepted that the video games weren't melting our brains). Usually, I played and she watched, but I did give her the controller... occasionally. I remember she did a lot of CoM after I admitted I sucked at the card system, ha ha.
Partly because it was something I did together with my sister, and party because its world-jumping, constantly changing, silly and convoluted and sometimes nonsensical atmosphere and logic just appealled to me, KH was my 'home' fandom as a teenager. I always came back. It was very bittersweet daty when, many years down the line, I accepted that I had finally aged out of it and just wasn't that interested anymore.
But that's fine, too. Since I don't care about the games or the characters like I used to, instead, whenever I think of Kingdom Hearts, I think about my little sister and the time we spent together.
We still harass each other with holiday well-wishes every year on Akuroku day and Zemyx day (both of them). And it's been twenty fucking years.
Like I said above, I was a KH fan as a teen, and ALL of the fanfiction I wrote for KH I wrote as a teen. These are the stories of a MUCH younger writer. I published some of them on a long-dead ff.net account, but the only longfic, 'Ashes to Ashes', I initially wrote in a pair of notebooks that went to school with me. I typed it up later, when I was a little older, so the later chapters of A2A are really the only things here with an ounce of traditional 'quality'.
Nothing here is very good... I'm just doing good by my teenage self, who was sure that everyone was going to love her fics once she polished them up and put them online.
Dream big, little Mouse. You're going to be hot among elf incest fans some day.
For what its worth, I did play some of the later games as an adult, specifically Days and KH3. But, for the most part, these were written in a world pre-Birth by Sleep. I remember I threw in some later elements while typing up A2A, but for the most part, all of these are the relics of an older time.
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Ashes to Ashes: Reincarnated versions of Namine and Roxas struggle to fit into modern day life on earth until they start to find the reincarnations of previous antagonists wandering the earth with them. Some people are delighted at the chance to be friends now; some are not. It slowly becomes clear that something else has followed them across time and space to their second lives, but this thing never died. Some Akuroku, some teenage lesbianism that never really got anywhere.
Don't Look Down: A series of previously-unpublished short fics that dare ask the question: what if Riku/Xehanort, a ship that barely exists even in fandom, was implied to be vaguely incestuous for no reason? I wrote these when I was a bit older (just post-undergrad, I think) as part of a monthly prompt series. What began as Riku resisting Xehanort's pull to the darkness swerved quickly into something about a mystically connected past between the two of them which connects them which Riku finds revolting and Xehanort compelling.