Final Fantasy in Kingdom Hearts

             Final Fantasy X and X-2 ~~> Auron (underworld), Rikku, Yuna and Paine (Hollow Bastion)

         Final Fantasy VIII:  Fuujin, Seifer, Raijin (Twilight Town)

Final Fantasy VII: Tifa and Serphiroth (Hollow Bastion, Sephiroth also in Olimpus Coliseum)

            Final Fantasy VI: Setzer (Twilight Town - Struggle)

                 Final Fantasy IX - Vivi (Twilight Town)

Final Fantasy X (Wakka, Tidus - Destiny Islands), FF VIII (Selphie - Destiny Islands, Squall - Traverse Town/Hollow Bastion), FF VII (Cloud - Olimpus Coliseum/ Hollow Bastion, Aerith, Yuffie, Cid - Traverse Town/Hollow Bastion)

Final fantasy VII Zack - Olimpus Coliseum, in BBS

                 I Think these are all FF characters in KH...