Kohryu The Iron Mole
-and-
Uranus The Chimera
If you want to unlock the other things hidden in the game look into Gamewinners.com or Gamefaqs.com data bases because iam not going to bother with posting them.
To unlockable The Iron Mole get to stage 5, without losing a single round in arcade mode. Kohryu will be your next opponent. beat him and hes yours.
Uranus the Chimera
Beat everyone in arcade mode without continuing. than you will have one chance to beat to beat her...believe this is no simply task...believe me..toook me like 10 time.
Shes harder then Shenlong as a secret character in BR2
Hint:
I really don't think there is really an easy way to beat her...I used Gado because hes the strongest in the game, but thats as much as i know GOOD LUCK.