in a interview with nomura, he said he wanted kairi to be playable in kh1 but that was scraped. and he wanted kairi to be in the spotlight for a while, if he did that kh2 would have been different with kairi as a main character. but kairi saved sora in kh1 from being a shadow forever. so he might be reusing those plans for the next story. having kairi be the hero and save sora. and yes riku. cus i believe only kairi has the power to save them from what is coming.