For the sake of all, here's the riddle reposted

I am a mystery to all, but one,
I work in the shadows, keeping my friend safe,
Such as the Yin Yang, we are opposites,
But we match so perfectly.

And my guess

How about ansem and Xemnas?

I am a mystery to all, but one, I work in the shadows, keeping my --> you don't know much about this two, only mickey knows.
friend safe,
Such as the Yin Yang, we are opposites, (ansem = heartless, Xemnas = nobody )
But we match so perfectly. ( they both want kingdom hearts and both are final bosses )