Beat Arcade Mode. The order that they are unlocked is as follows:

Unlockable	How to unlock
01	Kunimitsu
02	Bruce Irvin
03	Jack-2
04	Lee Chaolan
05	Wang Jinrey
06	Roger/Alex
07	Kuma/Panda
08	Kazuya Mishima
09	Ogre
10	True Ogre
11	P-Jack
12	Mokujin/Tetsujin
13	Devil/Angel
14	Unknown