There is a secret Audio track in Gallery mode for each character that can be unlocked by getting 100 in any of the characters Story mode. When you complete this task you will get a prompt telling you that the track has been unlocked in the Gallery.
This can be achieved on any difficulty setting with any number of rounds set per fight. When you then have at least 10 'Distortion' finishes and have not lost a round Unlimited Ragna will appear as the bonus 11th fight. If you defeat Unlimited Ragna you will obtain the 'Welcome to the Azure Nightmare' trophy.