Play as Testament
Go up to level 20 in survivor mode and you will fight Testament. Beat him and you can play as him.
Play as Dizzy
Go up to level 30 in survivor mode and you will fight Dizzy. Beat her and you can play as her.
Guilty Gear mode
Beat arcade mode with all characters to unlock Guilty Gear mode. You can also go up to level 100 in survivor mode to unlock this mode.
View character's ending
To view their ending, beat arcade mode with any character without using continues.
Alternate costume colors
When selecting your character, highlight a person and press Select to use his fifth costume; A + X to use their sixth costume; and B + Y to use their seventh costume.