1. Akuma: After unlocking Sakura Kasugano, Dan Hibiki, Cammy White, Fei Long, Gen, and Rose. First finish the arcade mode (on any difficulty) with any character, in this case use that same character for this second or later playthrough. Then fight Akuma on arcade mode, to do that get: at least 1 perfect round on 1-round matches (or 2 perfects on 3-round matches / 3 perfects on 5-round matches / 4 perfects on 7-round matches) including the last battle against Seth. No ultra combo finishes needed. But you can’t lose any matches/rounds nor use a continue when you die. Beating Akuma will unlock him.
2. Cammy White: Finish the arcade mode with Crimson Viper. That will unlock her.
3. Dan Hibiki: Finish the arcade mode with Sakura Kasugano. That will unlock him.
4. Fei Long: Finish the arcade mode with Abel. That will unlock him.
5. Gen: Finish the arcade mode with Chun-Li. That will unlock him.
6. Gouken: After unlocking Akuma, Sakura Kasugano, Dan Hibiki, Cammy White, Fei Long, Gen, and Rose. First finish the arcade mode (on any difficulty) with Akuma, and watch (not necessary to watch everything) an ending. In this case to play also use a character you already finished arcade mode with, and with which you watched the ending once, for his/her second or later playthrough. Then fight Gouken on arcade mode, to do that get: at least 1 perfect round on 1-round matches (or 2 perfects on 3-round matches / 3 perfects on 5-round matches / 4 perfects on 7-round matches), plus at least 3 Super Combo or Ultra Combo Finishes on 1-round matches (or 5 super/ultra finishes on 3-round matches / 7 super/ultra finishes on 5-round matches / 9 super/ultra finishes on 7-round matches), and at least 5 First Attacks — meaning you must score the first strike in a match — in 1 match (or 10 first attacks in 3-round matches / 15 first attacks in 5-round matches / 20 first attacks in 7-round matches). But you can’t lose any matches/rounds nor use a continue when you die. Beating Gouken will unlock him.
7. Rose: Finish the arcade mode with M. Bison. That will unlock her.
8. Sakura Kasugano: Finish the arcade mode with Ryu. That will unlock her.
9. Seth: Finish the arcade mode with all 24 other characters. That will unlock him.
Unlockable Japanese Voices: Beat Arcade Mode once and the option to switch to Japanese character voices will appear in the Options menu.
Unlockable Artwork: Complete the Combo Challenge for a character to unlock his or her artwork for the Gallery.
Unlockable Prologue and Ending Movies: Beat Arcade Mode with a character to unlock his or her ending and prologue movies in the Gallery.
Unlockable Colors, Taunts and Titles: Complete the following challenges once with any character to unlock the below listed rewards.
* Beat Time Attack(Normal)1 - Color 3
* Beat Time Attack(Normal)2 - Personal Action 2
* Beat Time Attack(Normal)3 - Title
* Beat Time Attack(Normal)4 - Title
* Beat Time Attack(Normal)5 - Title
* Beat Time Attack(Normal)6 - Color 5
* Beat Time Attack(Normal)7 - Personal Action 4
* Beat Time Attack(Normal)8 - Title
* Beat Time Attack(Normal)9 - Title
* Beat Time Attack(Normal)10 - Title
* Beat Time Attack(Normal)11 - Color 7
* Beat Time Attack(Normal)12 - Personal Action 6
* Beat Time Attack(Normal)13 - Title
* Beat Time Attack(Normal)14 - Title
* Beat Time Attack(Normal)15 - Title
* Beat Time Attack(Normal)16 - Color 9
* Beat Time Attack(Normal)17 - Personal Action 8
* Beat Time Attack(Normal)18 - Title
* Beat Time Attack(Normal)19 - Title
* Beat Time Attack(Normal)20 - Title
* Beat Time Attack(Hard)1 - Title
* Beat Time Attack(Hard)2 - ?
* Beat Time Attack(Hard)3 - ?
* Beat Time Attack(Hard)4 - ?
* Beat Time Attack(Hard)5 - ?
* Beat Survival(Normal)1 - Color 4
* Beat Survival(Normal)2 - Personal Action 3
* Beat Survival(Normal)3 - Title
* Beat Survival(Normal)4 - Title
* Beat Survival(Normal)5 - Title
* Beat Survival(Normal)6 - Color 6
* Beat Survival(Normal)7 - Personal Action 5
* Beat Survival(Normal)8 - Title
* Beat Survival(Normal)9 - Title
* Beat Survival(Normal)10 - Title
* Beat Survival(Normal)11 - Color 8
* Beat Survival(Normal)12 - Personal Action 7
* Beat Survival(Normal)13 - Title
* Beat Survival(Normal)14 - Title
* Beat Survival(Normal)15 - Title
* Beat Survival(Normal)16 - Color 10
* Beat Survival(Normal)17 - Personal Action 9
* Beat Survival(Normal)18 - Title
* Beat Survival(Normal)19 - Title
* Beat Survival(Normal)20 - Title
* Beat Survival(Hard)1 - Title
* Beat Survival(Hard)2 - Title
* Beat Survival(Hard)3 - Title
* Beat Survival(Hard)4 - Title
* Beat Survival(Hard)5 - Personal Action 9
Source: http://www.videogamesblogger.com/2009/02/15/how-to-unlock-all-street-fighter-4-secret-characters-guide.htm
