Birth by Sleep My theory: The dark Soldier is Xehanort!

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i kinda got a similiar theory to the one you posted, i never noticed the lack of yellow stripes on master xehanort's coat.

and i agree mostly.

 

I was thinking that the Dark Soldier was the first Heartless, because Master Xehanort said "Correct my mistake", and he was talking about darkness.

 

I really like the theory....and what if...Master Xehanort and Xehanort we know were related...like they were father and son....since they have the same hair color and eyes

 

There are a few too many flaws with this theory to work. For one The boss battle with "Terra" isn't canon. It never really happened. And Nomura Himself said it might not be Terra under that armor anyway. Plus Sora has never been to the room of sleep anyway.

And yes someone using darkness can use a Keyblade. Riku did it in KH1 and later in KH2. So it is possible

About the Dark Soldier having a Dark keyblade, Mickey's Keyblade came from the realm of Darkness so there can be a dark keyblade.
When you compared Riku and Dark soldier's suit, well that doesn't make sense. Yes the two are similar and show a connection, but Riku took that form BEFORE he was taken over by Xehanhort's heartless. And the heart unlokcer Riku had isn't a real keyblade so it shouldn't have a keychain.


Also most important, DS is too short to be Xehanhort