...ok i agree with some of that but let me point out a few things first just to say my share:
firstly @ lightning (prev page): they knew how riku fought because of CoM so... *slap ur logic* lol
next: the soul eater isnt exactly a keyblade, its kind of an it is but it isnt thing. it was just the weapon riku had found in the first game. otherwise why would he have even needed sora's keyblade?
next: if riku's been "helping" sora the whole time in KH2, why the hell would he decide to fight him and put his life in jeopardy?
next: the gang chased the "figure" sora fought all the way down the mountain into the city where xigbar was. also u dont exactly "know" that it was riku the emperor was talking about. all he said was that the guy was rude, which xigbar is definitely lol. and ya never know; maybe xigbar was going back to tell them it was getting closer or something. and sora called out to him thinking it was riku. honestly the only reason u dont actually see them chase him down the mountain is due to gameplay meets cutscenes but keep in mind they never actually lost sight of the guy.
next: back to the soul eater. would it really be so crazy to think that the organization made a replica of it? they did get enough data to make the clone so why not? the clone's soul eater was a replica after all.
next: and also if u pay attention while u fight the "stranger" lets call him, he doesnt exactly fight like riku does. riku is very deliberate and quick in his movements, whereas the person u fought holds the sword like riku, but then uses it rather clumsily and carelessly, in a casual manner.
and just to put to rest a question about my earlier comment of : he could be warning them again,
remember, the organization has no real love of the heartless, nor are they inherently evil (e.g. axel, roxas) they all just play the game so to speak and seek to be whole. they arent actually hurting anybody, they just kind of toy with the heartless while sora keeps killing them, which is what they want to begin with.
*pants, too much typing lol* i think i covered everything lol. if i am somehow indeed wrong, then the KH crew put some serious holes in this fight/ chase/ another fight sequence.
almost like the holes in dot hack: GU's storyline lol. but im not goin into that here xD
oh one more note!: i think at the end of CoM the soul eater becomes the way of dawn, since riku kind of mentions it when Diz asks him what path he will take. idk that could be nothing but figured i would mention it anyway.
EDIT: oh ya and final final note i swear lol:
nobodies can use dark powers lol. thats how they AND riku AND DiZ were moving around with their little swirling portal thingies lol. and roxas even says he's "someone from the dark" and the bio even says : keyblade wielder and denizen of the dark.
*SUPER SHOT ALL YOS LOGIC* again if i am somehow indeed wrong about all of this, then the KH crew really effed up here! lol
EDIT2: ok guys i even just watched a video on youtube of it. now unless the camera subtracts like 40 pounds, the guy i just saw sora fight was super skinny, like xigbar skinny, with skinny arms and slightly broad shoulders, just like xigbar's. soooo... yeah to me that pretty much sums it up (along with all of my wonderful above points lolz)