To perform this cheat you need two memory cards.

Copy your file on to the second memory card and go to Card Trading. Trade as many cards as you wish from memory card 2 on to memory card 1 and voila more cards!

To get Caitlan, complete Rusty's story
To get Earl, complete Trey's story
To get Jalen, complete Rebecca's story.
To get Knight, complete everyone's story.
To get Marcus, complete Flynn's story.
To get the monks, you must defeat them the first time.
To get Sera, complete Luc's story.
To get the Widow, defeat her at the end of the tournament.