Guilty Gear X: Advance Edition

Alternate costumes:
Press Start or Select when choosing a fighter at the character selection screen.

Fight as Dizzy or Testament:
Press Down, Right(2), Up, Start at the "Press Start" screen.

Fight as Testament:
Press Up, Down, Right, Left, A at the "Press Start" screen.

Fight as Dizzy:
Press Down(2), R, L, R at the "Press Start" screen.

Original mode:
Press A, B, A, L(2) at the "Press Start" screen.

Limit release mode:
Press L, R, A(3) at the "Press Start" screen.

All Extra mode fighters:
Press Right, Down, Left, R(2) at the "Press Start" screen.

All Guilty Gear mode fighters:
Press Left, Down, Right, L(2) at the "Press Start" screen.

Fight as Dizzy:
Defeat the Dizzy's dare devil version in survival mode. Alternately, defeat Dizzy on stage 10 in arcade mode.

Fight as Robo-Ky:
Enable the "All Extra mode fighters" code. Then, choose Ky Kiske in his extra mode. Alternately, unlock Extra mode for Ky Kiske by defeating a fighter's Dare Devil version in Survival mode.
Information in this section was contributed by Louie.

Fight as Testament:
Defeat Testament's dare devil version in survival mode. Alternately, defeat Testament in stage 9 and Dizzy on stage 10 in arcade mode.

Extra mode:
Defeat a fighter's dare devil version in survival mode to unlock extra mode for that fighter. The fighter's extra version is able to execute new moves and supers.

Original mode:
Successfully complete the game in survival mode to unlock the "Original Mode" selection at the options screen.

Guilty Gear mode:
Select survival mode. Get to stage 180, then continue on and defeat all opponents until reaching stage 330. The Guilty Gear versions will now be unlocked.

Hint: Tougher opponents:
To fight difficult opponents, enter training mode. After choosing your characters, pause game play and go into the opponent settings. Change the mode of opponent from "Free" to "CPU". Also, change level of the CPU to "8". If you turn everything off that remains in the settings (regaining health, etc.), you will be able to fight the opponent in training mode just as in arcade mode. You can also change to tag and 3-on-3 in pause options. The CPU will now fight very hard.
Information in this section was contributed by JEDIKEVIN2.