NOTE: My internet as been wonky as all hell, randomly disconnecting me and connecting me as I tried to do thing like type in the Captcha or agree to the new forum rules about the health bar, so, if this end's up being a double post (or multi post at this point), I apologize, and I'll fix it as best as possible when I see it.
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The thing is, at certain points in the game, Sora matches, and even surpasses, Mickey. In Ultimate Form, with his ability to Telekinetically control one blade while wielding another, his power pretty much matches Mickey. And in Ultimate Form, that Twilight-like blending of light and dark, he's pretty much the most powerful person in the game. Consider this; Xemnas is easier to defeat in Hard mode while Sora is level 71 if you are in Final Form than if you are playing on normal mode with Sora at level 99... and you're not in Final Form. That's how overpowered Final Form is. Riku is similar in the GBA game, except using only darkness (and then destroying everything).
That also makes sense in the KH universe; Kairi is light, Riku is Dark, and Sora is Twilight. Mickey uses pretty much exclusively light, and so his power tops out far before Sora's, who wields everything, or Riku, who is naturally a light being, but has submerged himself in the dark.
On the other hand, Mickey's power is always there. He's always at 100% power, with nothing holding him back. Neither Sora nor Riku can do that; to use their full power, they need to harness outside energy and force themselves into alternate states.
Oh, and yes, Kingdom Hearts was pretty much the first time I ever cared about Mickey Mouse, because he's an asolute badass in those games. Though, funnily enough, i didn't know he would save you until my second playthrough, on Normal (I plyaed it on Hard first because a few friends had told me that the game was a lot easier then the first one. The Normal run was just to mess around in for fun). I had kept myself underpowered for the hell of it, and did fairly bad on the boss in Mulan's level, so I said "Screw it. I'll just let myself die and start the battle over." And then Mickey showed up. I was startled at that.