For many years, people have insisted that the relative flatness of Kairi's character is a flaw in the storytelling of Kingdom Hearts, but honestly, I disagree. That flatness is at the heart of the narrative-- the simple, supreme power of unconditional love is, by its very nature, uncomplicated. You don't need to jump through the hoops of social theory to understand why a girl like Kairi would be very close to the heart of the world.
BUT that flatness comes with other dimensions. You can't have power without sacrifice, and Kairi's power to save others comes at the cost of being literally imprisoned and being denied common avenues of personal growth.
See, light is the power of love, but darkness is the power of desire, that which must be obtained in the world beyond. Even though it's darkness that attracts others to Kairi's light, it's also darkness that pulls them away from her. This is the root of her fear of abandonment. Kairi longs for company and security that won't run away when something more interesting appears on the horizon, but she also is fundamentally restricted from any act that would keep people near her.
This is why Naminé's a "special Nobody". There is no darkness to form a Heartless, no secret drive that can power her beyond the estrangement of heart and body. The only things that belong to her alone are things that can't be changed anymore: memories, the foundation of love. She can only control people by keep them fixated on her the same way she is forced to fixate on them.
In short, the argument of Kairi's character is that there is no fate imaginable worse than being a Disney Princess: in a state of perpetual perfection, as ordered by the world itself, without hope or despair. You have been made the vessel of a love far older than yourself, and then punished for containing it. Even your most estranged self is ultimately just a broken mirror crying alone in her room.
Kairi isn't being written in a misogynistic way, she is an intentionally placed victim of cosmic, in-universe misogyny: the cage placed around the personal, sexual, and reproductive potential of those considered too useful to be free. She's the KH answer to the intellectual framework of the world of Disney: a real princess stranded from the "perfect world" where establishing women as a class isn't oppressive.
Kairi is what happens when Sleeping Beauty wakes up, leaves the kingdom forever, and finds herself lost because she was never taught how to want for herself.
