Capcom released their Street Fighter IV Training Manual. Click here and download the SFIV Training Manual.
Alternative download link.
So how do you unlock all the characters in SFIV? Beating the game in Arcade mode with certain characters unlocks new characters.
Ryu >>> Sakura
Sakura >>> Dan
Chun-li >>> Gen
Abel >>> Fei Long
C. Viper >>> Cammy
M. Bison >>> Rose
In order to get Akuma (Gouki):
- Sakura, Dan, Gen, Fei Long, Cammy, and Rose unlocked.
- Play Arcade Mode with a character you've already beaten Arcade Mode with.
- Don't lose any rounds.
- Get the same number of Perfects as the number of your rounds you're playing. e.g. 3 Perfects for 3 rounds per game.
- Beat Seth.
- Akuma will appear.
- Beat Akuma and he will be unlocked.
- Beat Arcade mode with Akuma.
- Watch a bit of the ending movie.
- Play arcade mode with Akuma again.
- Do not lose a round.
- Get the same number of Perfects as the number of your rounds you're playing. e.g. 3 Perfects for 3 rounds per game.
- Perform a certain number amount of Supers and Ultras to correspond to the number of Rounds. 3 (Supers/Ultras) : 1 Round Games
- Get "First Attack" 5 times.
- After you beat Seth, Gouken will appear.
- Beat Gouken and unlock him.
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[Source: Sony PS3 Modding - Homebrew, Upgrades, Mods, and Hacks]