You fight Demyx in Hollow Bastion, not Twilight Town. :'D
And I'm pretty sure you fight the 1000 Heartless BEFORE him, because you still have Goofy with you. After Demyx, something happens to poor Goofy.
Did you play the first Kingdom Hearts and MAYBE Chain of Memories? D:
If you didn't, you'd be missing out on a LOT of story!
I've owned and played every single Kingdom Hearts game, and the whole series is loads of fun~ The story gets pretty deep, the gameplay is fun, and the characters are enjoyable. Not to mention the ability to interact with Disney characters makes it even more fun!