I hope it's okay for me to add my own two cents. It's as if the writers at some point in the last 5-7 years that Kairi wasn't suited for combat. The only thing I can figure is that they didn't really know where her story was going. I'm beginning to think that the lead writers can't write a female character without making her have a tragic story. Xion, Namine, Aqua and now Kairi. Yeesh.

It was important that Kairi was trained if for no other reason than the 15 years of buildup from KH3. In that time period, I went from my 20s to literally having grey in my hair. That's a very long time to set up that kind of story. To me, it ultimately feels like a waste of the audience's time. As I stated before, it would have been fine for her to remain a spunky non-combatant but with her special Princess of Heart powers. Instead the writers decided to give her a token appearance in the final battle. Like they knew that they had set this up, but had no intention of actually having her in the final 'Nort conflict. That's also without mentioning the complete and total Flanderization of her character in KH3.