Cloud = Nicky-dah (nickname for my blonde-haired brother)
Tifa = Ellie (my dog's name)
Barret = Marcellus (of Pulp Fiction fame)
Aeris = DeadChick (I certainly hope that's not a spoiler...)
Red XIII = Big Dogs (it's an inside joke, I know he's not a dog)
Cid = Bad-ass (pretty self-explanitory)
Yuffie = DumbBi*ch (again, self-explanitory)
Cait Sith = CatAndFat (I never used him anyway)
Vincent = Darkness (no, not Charlie Murphy)