Guilty Gear X (PS2)

Unlock Testament and Dizzy:)
At "Press Start" screen, press Down, Right(2), Up, Start to unlock Testament and Dizzy.

Or

Complete the game in arcade mode or reach level 20 in survival mode then defeat Testament. Reach level 30 in survival mode and defeat Dizzy to unlock her in all modes.

Guilty Gear mode:)
Reach level 100 in survival mode and defeat the last opponent to unlock the Guilty Gear mode option. Alternately, unlock Testament and Dizzy then successfully complete arcade mode without losing any rounds with the time option set at 60 seconds. This may be done with any difficulty and round number setting.

Alternate costumes:)
Hold Start, Triangle, or X when selecting a fighter at the character selection screen to choose three alternate costume colors.

Play as Mecha Ky:)
Pick Guilty Gear mode and choose Ky at the character selection screen.

Use a Special attack (any character) :)
Press Triangle + Square during a match

Playing as Ki Kiske:)
After gaining Guilty Gear mode, Ki Kiske is the only one truly affected. He trips over his words, crawls on the ground, gains a new attack and alters two attacks. The Down, Back + X and Down, Forward + Circle attacks are different. Additionally, push Forward, Down, Forward + Circle with half a limit bar, and he will do his combo attack upwards.