Street Fighter IV

Here is how to unlock all secret fighters. To make this easier, set the difficulty to the easiest setting and set the number of rounds to one.

Cammy - Beat Arcade mode with Crimson Viper
Fei Long - Beat Arcade mode with Abel
Gen - Beat Arcade mode with Chun Li
Rose - Beat Arcade mode with M. Bison
Sakura - Beat Arcade mode with Ryu
Dan - Beat Arcade mode with Sakura
Akuma - Unlock all characters abover then start a new Arcade mode with any character. Set the match to a single 30 second round and beat the game using NO continues and have ONE perfect round. After Seth, Akuma is next, defeat and unlock.
Gouken - Unlock Akuma and start a new game again with any character. Set match settings to a single 99 second round and beat the game with: 2 perfect rounds, 3 Ultra or Super finishers, 5 strikes and NO continues. After Seth, Gouken is next, defeat and unlock.
Seth - Beat Arcade mode with all 24 unlocked characters.