To unlock you must simply complete the story mode beating all eight opponents using Aganos on any difficulty earning any objectives.
If you want progress towards :
He has three objectives, and all can be completed in one run.
1) Beat story mode. Simply win all eight fights.
2) Combo Break every opponent. When you are on the receiving end of a combo, you need to break it against very opponent using a combo breaker. You need to use the same break that corresponds to strength of attack being used against you.
Light attacks :

+

Medium Attacks :

+

Heavy Attacks :

+

You need to do this as the hit connects against you. If you do it successfully, you will break their combo. If you fail, you will get locked out. Do it once per match successfully.
3) Ultra combo Thunder. Simply beat Thunder with an Ultra in the seventh fight. His ultra is



+

when Thunder's health is reading 'danger'.