Quote Originally Posted by EoFF
Jack - Walks in front of the elevator; usually from the Library.
Knight Club - The curving corridor with exits to the Dormitory, Parking Lot and Cafeteria.
Princess Diamond - Two female cadets in front of the Directory in the Lobby.
Prince Spade - Second floor, corridor leading from the elevator; he's the guy who gave you the cards near the beginning of the game.
Queen Heart - At the bridge; it's Xu. You can get the Carbuncle card from her.
Former King - In the infirmary; this time it's Dr. Kadowaki.
Master King - In the Dormitory; choose to sleep and the Kingwill wake you in the middle of the night. It turns out to be Quistis! Defeat her for the Gilgamesh card.
Joker - Inside the Training Center; he's the guy who sells stuff in there. He'll give you a Battle Meter upgrade, and you can also win the Leviathan card from him.
Copied from EoFF frontpage. I've found Jack to appear more often walking from the infirmary to the elevator, though. Also, it's not required to challenge Joker to challenge the King.