|Funny thing is that it's supposed to have ControllerPak save, however nothing happens when you hit the save button under options.
To unlock each secret, move to each character listed and tap start after each one, then go to your desired character and select as usual. All of these secrets are for two-player mode except for the Random A.I.
Battle in a Mini Golf Course (Koyasha, Mordus Kull, Takeshi)
Battle at Machu Pichu (Namira, Koyasha, Taria)
Battle in the Castle (Mordus Kull, Taria, Ragnar)
Battle in San Francisco Rush (Xiao Long, Al' Rashid, Koyasha)
Battle with a Big Noggin' (Ragnar, Al' Rashid, Takeshi)
Battle in Micronaut Mode (Takeshi, Al' Rashid, Ragnar, Xiao Long)
Battle in Super Speed Grid Mode (Ichiro, Xiao Long, Koyasha)
Battle the Random A.I. (Hell Knight, Xiao Long, Dregan, Namira)
To select a particular stage in which to fight, go to the character's portrait in player select and tap START four times. Then select your character as usual.