Every time Kingdom Hearts series is mentioned online, someone will say: "Even if you played all the games you won't understand the story." That or something along these lines.
While I think that can be true for some people, especially those who don't want to think about the story to understand it, I believe that if you do pay attention, you won't have to play all the games in the series to understand the story!
Kingdom Hearts franchise is a cross-over between Disney and Square-Enix properties. In which original characters travel between worlds based on Disney and Final Fantasy to stop the darkness that strives to destroy all the worlds. The main story follows the adventures of Sora and his friends, with most spin-offs providing back-story and context for Sora's main adventures.
For newcomers, Kingdom Hearts series is confusing: There are currently 10 titles in the series excluding expansions & updated re-releases. Most of them have strange titles that even fans have trouble making sense of. (Who names a game 358/2 Days?!) All of the games in the franchise add to the story and lore. If you miss any of these games, you wouldn't understand the references in other games. That makes the series confusing even before you start playing it!
But here's what I believe: I don't think Kingdom Hearts story is hard to understand, I do think it's hard to explain! If you paid attention, the story is actually easy to follow.
The first thing you have to put in mind is that you don't need to understand why everything is happening to enjoy it! Sora, the main character does not understand any of it, and he still fights for the right thing!
The second thing to out in mind is that the story is not written to be logically coherent. The logic is secondary to the emotional value in Kingdom Hearts games. The story is logically coherent, that's why it works, but if you disregard the emotional part of it: the story becomes needlessly convoluted!
I believe Kingdom Hearts fans who love the story as whole are two types. (A) The ones who don't mind the story at all and go with the flow. (B) The over-thinkers who have made theories over the complicated parts of the story.
So, if you follow Kingdom Hearts story and could understand what is happening in the moment and what it led it to happen that way, without trying to link up what's happening to the lore, then you're understanding what you should understand from the story.
I know what I said is a paradox! How can I understand what led to something happening without linking it to the lore? ...Well, because it becomes more confusing the more layers deep you get into... It does follow a logical sense but it's too long that you might hamper your enjoyment of the scene as you watch it.
If the only thing you need to understand to follow Kingdom Hearts II is Sora and Roxas connection, and you tried to explain KHII in terms of Ventus connection to both of them, and then you dive into Vanitas who looks like spoilers, then Kairi comes into the mix because Sora removed his heart and... (I'll stop here.) You'll eventually get the full picture of the story, but that requires time and you should be enjoying Kingdom Hearts II actual story at that time, you'll have time to think of it all after finishing the series.
That's why I said Kingdom Hearts is easy to understand but hard to explain... You can enjoy it at the surface level. You'll probably understand much more than the main characters combined. There's also a lot of emotion there. You can dive deep into it, but you'll only find the enjoyment in that if the act of diving is enjoyable to you, because there's no reachable Seabed as of yet.
If you're the type who enjoys diving deeper, then there's no stopping for how much you can theorize with Kingdom Hearts franchise. The story is written for these types of people, in a way that those who only see the surface level stuff can enjoy it.
I'm not saying that KH has a deep story, because arguably there isn't necessarily any deeper meaning behind all these layers. I'm saying that the existence of these layers in the first place is for you to enjoy analyzing them. The reward is having a better idea on the lore itself.
Kingdom Hearts is like a puzzle and the pieces of that puzzle is the series entries. Some parts of the puzzle are essential, miss one part and it becomes unsolvable. Other parts are not so important but they paint a different image as while.
What makes it more appealing as a franchise, is that with every new entry released, the puzzle itself is changed to incorporate this new piece. Making this a never ending puzzle. Kingdom Hearts III's ending is just the beginning of a new saga!