After rewatching the cutscene that introduced Yozora to Sora, I kinda got hit with a epiphany, though again, this is just a theory, A GA- (yeah right)

So Sora has been messing up Worlds, in his naivety of being a tool for the Organization and his constant failings over the years, done enough to mess up Parallel Universes(Worlds). 

Yozora is a someone from one of those Parallel Worlds, and his mission is to defeat Sora, although from having gone through the lanes between Worlds himself to find Sora, his one memories and possibly his heart has been effected as well, giving him his uncertainty about who Sora exactly is. Please let me know your thoughts! Again this is just a guess really

well it kinda conflicts with that fact that he recognized sora and he said he was going to save him

I mean Parallel Universes are the one thing nomura has yet to bring into the franchise as incing on his very confusing cake. 

I suspect something like your theory to be true yzora being another version of sora created because sora using the forbidden technique has now messed up time and space