Ah, the Kairi thing. Of all the ways DDD dropped the ball, I think this one gets at me the most, as there really is no satisfactory in-universe explanation.
The flaw with the idea that Kairi has no combat experience is that it's centered around all of what we don't see as players, and contradicted by much of what we do. On Destiny Island, there are two primary reasons we don't see Kairi practice fight which have no bearing on whether she's capable of it, or whether she regularly participates:
1) During the time we spend on DI, Kairi has taken charge of compiling an inventory of supplies needed to build the raft, and also appears to have busied herself watching over the work which has already been completed. So she's otherwise occupied.
2) Even if she were participating, I think it goes without saying that Sora wouldn't fight her. That doesn't mean other characters didn't.
The first point goes more to a convenient excuse not to show her fighting, which would be in keeping with Nomura's desired narrative (from Sora's perspective): that of Kairi as a pure girl who needs protecting and who is made vulnerable as a function of the plot's design and his hero's journey. It's a lot harder to sell that on a character (especially a female one) who has been seen to kick butt. This doesn't stand for evidence that Kairi did do combat on DI, it's just to offer a plausible ulterior motive for neglecting to show it.
But wait, there's more. First, there's KH2, where Kairi is actually shown to do combat at the spur of a moment when given something to swing. Sure, one could make the case that any ol' person could knock a few Shadows around, but that actually doesn't follow from our given point of reference: Sora, who has prior "combat" experience (via training on DI) when he first gains the Keyblade and fights a bunch of Shadows on the night of the storm. So what we can actually take away from her fighting in KH2 pursuant to the internal logic of the series is that she's no less experienced than Sora was at the start of KH1.
But wait, there's more! KH1 comes with a twist which changes, well, everything about the narrative that Nomura intentionally set up. Kairi was with Sora the whole time. That's, like, the point. Her heart was with him and she experienced everything he did on his journey. Her heart was awake inside of him and it was actively responding to different stimuli, causing him to have feelings he couldn't explain and see visions of her where she couldn't have been-- physically, that is. Her journey with Sora is etched into her heart as links in her chain of memories. She fought with Sora, and grew with Sora: she was as much a part of the narrative of KH1 as he was.
So she was there in KH1, on the same journey as Sora. She was there in KH2, on a journey of her own: one which culminates in her earning the Keyblade for herself. That's 2/3 of Sora's stories. Can somebody tell me again why Kairi doesn't qualify as having enough "experience" to take part in the MoM exam, even if she wasn't aiming for the Mark itself? It can't have anything to do with her physical prowess: that discounts all of Sora's fighting from the moment Kairi revives him (oh yeah, that was Kairi, the useless character who would have been totally useless on a mission that has to do with moving between Realms of existence) to the moment Roxas joins up with him at the start of KH2 when Sora had no body: 'cause it's strength of heart that really matters in this series.
Kairi's heart has loads of things going for it. It has experience being removed from her body and put back in one piece: it has all of the memories from the time it spent with Sora in KH1: it has, y'know, the no darkness thing: and it has the resolve she built steadily over the course of KH2 buoyed by her singular, powerful, guiding connection with the Hero of Light which has saved him more than once throughout his journey.
She's the worst twist in DDD because it was obvious that she should have been there from the get-go. It makes absolutely no sense that she isn't, because her heart quite literally is Sora's "guiding key." It makes everybody there look like total amateurs: Yen Sid, Mickey and Riku of all people should have insisted on Kairi's presence and participation, and it's egg on Nomura's face because it wasted a perfectly fine opportunity to deepen our understanding of her purpose and her powers.
