Based on what I've seen so far:

Lightning - Cloud or Ashe.
Snow - Seifer, with a bit of Zell.
Sazh - Barret, I guess.
Hope - a less whiny version of Tidus. Yes, I mean it, less whiny.
Vanille - Rikku. Maybe a bit of Penelo.
Fang - I haven't used her much yet, but I'd say Paine, or maybe Auron.