I have heard several things and here is what I have put together.
One of the hooded figures is evil (the dude with the tricked out heartless) the other is Riku. The evil hooded figure has been called Unknown by many people just as a nickname (similar to WEAPON of FFVI being called Sapphire Weapon). The guy in lj's sig is Unknown and he is looking for Sora to destroy him as he is the only thing that can keep darkness from overwhelming the worlds. The girl on the beach is Kairi. Her skirt says it all. The only thing I don't get is how they all got keyblades.
