Maybe the Beast?

I am a shadow of what I once was (Guess you know the beasts story)
Once I was something (A prince)
Now a mere replica ( Becoming a beast)
Of something greater

But do not reject me (Don't fear the beast)
For I will stand by you (It helps you)
when you have nothing
to hold off the shadows (In KH I when you are in hollow bastion and having nothing but your wooden sword)