I kind of feel like at this point if Sora and Kairi have a simple not-complicated relationship it is because there is a lack of development and the relationship (and Kairi) isn't really fleshed out, not because they have an open and trusting relationship. Like, look at Sora and Riku, you can say they have that kind of relationship now, by KH3D, but it took 7 games to get there and they both had to develop and overcome a lot. Kairi and Sora are more or less exactly the same as they were in KH1. They haven't been given that chance to have conflicts and grow. Any chance for conflict + friction is simply side-stepped (Sora telling Kairi she'd get in the way, for example). Kairi isn't really allowed to have her own motivations and feelings about things. She isn't given space to explore how she feels about various plot developments, and definitely not in the way that Riku has. She says at the end of KH2 that she isn't going to be left behind anymore, that wherever one of them goes, the other follows. But then cut to Blank Points and suddenly she is saying goodbye to them. There is nothing showing why she made this change, or her dealing with it, or her being upset with being left behind again or anything like that. She's just fine with it, because the plot wants her to be fine with it. It rings hollow, like Kairi isn't really her own character, her behaviour simply changes to fit what Nomura wants instead of it being consistently developed.
Even if she wasn't going to take the Mark of Mastery or anything like that, for example, why couldn't Kairi still have gone to the Mysterious Tower? Donald and Goofy and Mickey spend like 90% of the game just sitting there talking with Yen Sid. Why couldn't Kairi be a part of that? Why couldn't she wait for her friends to return from there? To be apart of the celebration? If Nomura really was interested, he could have given Lea's story in that game to her, even. (Or have them both do it). It's just, agh. There are always opportunities for Kairi to be more involved in the games, and they just side-step them. It's infuriating. Like, can you imagine how cool it would have been if, after seven games, we got to see a scene like this:
"She made it!" -- KH AU in which Kairi is the one to save Sora.
bleh, sorry i guess I'm drifting a little with the topic. It's just that Kairi is consistently denied the opportunity to be involved and have agency (and even presence!) in the story, and that's likely while SoKai as it currently is seems so simple, because she has hardly been given the chance to be a character at all.