To play as this charachter beat the game in default difficulty in ratio mode. After the credits you will see a message confirming you did it.
Play as Shin Akuma:
Beat the game in either ration mode or 3 on 3 mode. If your groove points are high enough, you may get to face Shin Akuma. Beat him (if you can) and he will become a playable charachter.
SubBoss and Boss Fights:
Before the final battle, you may get interrupted to face one of two subbosses, either M. Bison or Geese Howard. This will only occur if you have over 700(?) groove points. After the final round you will fight one of four bosses if your groove points are high enough. If you have over 800(?) groove points you will at least face either Akuma or Rugal. If you have over 1000(?) points you may get the opportunity to face either Shin Akuma or Ultra Rugal.
Special Starts:
When two charachters start off a round against each other, they may perform a special pose or action. I have discovered 14 different ones so far.
Ryu VS Ken Rock VS Geese
Bison VS Iori Hibiki VS Rolento
Chun-Li VS Yamazaki Ryo VS Yuri
Terry VS Geese Kyo VS Iori
Ryo VS Kyo Terry VS Ken
Bison VS Geese Ryo VS Sagat
Sakura VS Yuri Raiden VS Zangief