Street Fighter 4 - Unlock Akuma

Unlock Gouken:
First, unlock Akuma, Sakura, Dan, Cammy, Fei Long, Gen and Rose then Fight him on arcade mode. Get at least 2 perfects and 2 Ultra Finishes to unlock him.

Unlock Cammy:
End the arcade mode with Crimson Viper

Unlock Dan:
End the arcade mode with Sakura

Unlock Fei Long:
End the arcade mode with Abel

Unlock Gen:
End the arcade mode with Chun-Li

Unlock Rose:
End the arcade mode with M.Bison

Unlock Sakura:
End the arcade mode with Ryu

Unlock Seth:
End the game with all the other characters