i am a litile confused
when xion was in the castel in the world that never was
she dremed when zexion and riku was at destiny island
they where alone nobody was with them how did she knowed about it?????
I dont think it's ever explained, but one theory is that since she is in effect a mutated replica that is altered by memories it sucks in, and she was hanging around inside the memory-sensitive Castle Oblivion, she reacted somehow to Riku's memories.
But that doesn't explain why Roxas seemed to be there too.