And water is wet.
Everything you have stated are only facts, none of that is character development.
No, that's actually not "development".
Those are just things she is.
It's like, I can introduce my own original character and spend half an hour naming its titles, like "King of the West" "Vanquisher of Toothbrushes" and so on, but that doesn't develop its character at all. They're just statements about it.
Character development is defined as "
The process of creating a character's background, physicality, appearance, and personality".
Everything that I mentioned earlier fits into that category from an objective standpoint. I'm not saying that Kairi had good character development, I'm just saying that she has character development whatever little there is. Acting as though she has no character development at all is baffling to say the least.
For example, knowing that Kairi is a Princess of Heart means that she will have a connection with the other Disney Princesses. That’s a trait of her character. She’s a Princess. Yes, that’s a statement. But that statement is a formation of her character. Give a character enough adjectives to describe them and you’re able to piece together said character’s personality. Personality is a factor for character development.
Kairi on the other hand has been prominent in multiple games yet faild to develop character traits that don't revolve around Sora or her status as a PoH.
That's barely more than stated facts. Being someones love interest or someones somebody...how is that development?
Actually, in BBS, we see that Kairi originated from Radiant Garden as a child. That’s a development into a character’s past thus being classified as character development because we are delving deeper into said character by seeing them in different era’s.
And believe it or not, a love interest is an aspect of character development beyond a trope. Sure, it can be argued that’s nothing more than a trope, but it’s also a relationship dynamic. A relationship dynamic that affects the protagonist of the franchise (Sora), and ultimately, how the audience perceives Kairi in the narrative of Kingdom Hearts.
Again, I’m not saying that Nomura did a good job on Kairi's character development, I’m just saying that she has development. Just minimal development. There’s no way in arguing against that objectively. No character development means that we know "nothing" about the character outside appearances, and clearly, that's not the case for Kairi.