Tbh, I don't really like Kairi's treatment in the series, either. But I like her being a Keyblade wielder, because she said she always wanted to fight, and at least it's giving her
some role. Frankly, I don't like the idea that she has to be a fighter to be important, but in a fast paced series like KH, I guess I can get why they'd think they had to do that. I just wish Nomura would have planned out a more substantial role from her early on. Especially since she was such a driving force in KHI (even CoM somewhat), but now is just disappearing more and more. It's a pity; especially since I feel like they had more planned for her during the time of the KHI beta trailer.
And mostly Kairi's Aqua's "successor" because Aqua did give her the power, but other reasons, too. Like how they have similar traits, both have hearts of "pure" Light, might end up walking the same path, etc. Sora and Riku walked similar paths to Ven and Terra, so Kairi might do the same with Aqua. I already think she has a little bit, actually.
I swore I'd never do this, but I actually wrote a fanfiction recently that kind of shows how similar Aqua and Kairi are in ways, and that really all Kairi's missing is character development. I usually wouldn't post this, but it is my best work (so I'm not
completely self-conscious about it), and I think it shows a lot of things that people usually miss between the two. Here it is,
One Sky, One Destiny, a kingdom hearts fanfic | FanFiction if you want to give it a read. Any constructive criticism would be greatly appreciated, since I want to be an author and am working towards that goal.
About the punching Riku thing: do you remember the scene towards the end of KHII when the gang is celebrating their "victory" over Xemnas (right before Roxas and Nami return to Sora and Kairi, and all that)? Well, if you zoom in on that part, you'll see that Kairi actually slaps Riku in the arm. I always liked that scene, because I actually thought it was a rather cute, rare Riku/Kairi scene that maybe showed their dynamic with each other. Anyway, I only know about it because I accidentally found it while video editing once. I'd post it here for you all to see (as I have a gif of it on my PC), but I don't really know how to do that. If someone can tell me how, I'd be happy to post it