Yuna, Rikku, Payne. That's it. You'll also see other characters from the original FF10, but they won't be playable.
If the story is as good as FF10's story was, FF10-2 will definitely be worth playing. My only concern is the battle system. If it's like Kingdom Hearts non-turn system, I'll probably hate it. If it's the traditional turn-based system like all the other Final Fantasies, I'll love it!
Have you guys seen the trailer? It looks awesome!