Sora does change. In all of the ways that count and hurt, he changes over the course of this journey.
They’re sitting on that pier, looking at this beautiful sunset, and Kairi’s a little uncertain but she’s mostly excited–she’s ready–she’s going to go on an adventure, and she can’t wait. It’s not scary, because “I know I can always come back here, right?” Riku’s desire to see the outside world has wormed its way into her heart.
But Sora–he’s actually the one of the three who doesn’t care so much about the quest. He loves his friends and he’s excited for the trip because it’s going to be a great adventure with his two best buds, but that’s the only reason he’s leaving. He doesn’t care about the outside worlds; in that sunset talk they had on the paopu tree, he offered that nonchalant “I dunno”, sat back, and disconnected from the conversation about the outside world until there was a lull and he had the opportunity to ask Kairi a question. He even rolled his eyes at one point. He doesn’t understand Riku’s wanderlust; he loves their island with all of his heart.
Then the world falls to darkness, and everything changes.
No friends. No island. Sora is alone in the outside world that he didn’t even care that much about–what’s the point of exploring if he’s not with his friends? If he can never, ever go back to the island that he loves? And this despair affects him–he’s not at all as carefree or go-with-the-flow as he was on the island. He holds onto his determination and drive, but he gets angry easily, he glares and he snaps at everyone and he and Donald can’t have a civil conversation to save their lives. And when he can’t muster enough anger to cover his hurt, he’s just so sad. Yeah, the smile he offers Donald and Goofy in exchange for going with them is a funny moment–but considering that he was ninety percent smiles before this all happened, the fact that he can only muster a garish mockery of a grin is a little heartbreaking.
He changes, in all of the ways that Kairi doesn’t want him to change.
The story, for his character development, is about that change being reversed, It’s about healing that hurt, getting him excited about all of the travelling he’s doing and the friends he’s making, putting hope back in his heart that the loss will be reversed. You can bet that KHII Sora would’ve been able to fly in Neverland on his first try, but KHI Sora? Happy thoughts and faith are just so hard for him, even that far into the story.
By the time he gets to Kairi again, he has learned and grown and healed so much that, for all intents and purposes, he hasn’t changed. He’s the carefree boy she grew up with and loves. But he’s infinitely stronger than that boy, because he had to lose and find himself again.
So this moment, where they’re both happy and content, Kairi’s profound foresight makes me so happy. Sora doesn’t understand, but Kairi knows him well enough that, even if she doesn’t know what’s coming, she understands that the most important thing for him is staying strong and not losing himself.