These tips are pulled from IGN IGN's tips

Hey just got Street Fighter 4 and don't know how to unlock your favorite characters like Dan and Rose? Well, don't worry I have you covered. So far I've unlocked everyone but Akuma, Gouken, and Seth but that's because getting them is a bit more complicated. Anyway here you go here's how to unlock the hidden characters.

Unlock Cammy: beat arcade mode with C. Viper
Unlock Fei Long: beat arcade mode with Abel
Unlock Gen: beat arcade mode with Chun Li
Unlock Rose: beat arcade mode with M. Bison (Dictator)
Unlock Sakura: beat arcade mode with Ryu
Unlock Dan: beat arcade mode with Sakura
Unlock Akuma: First, unlock Cammy, Fei Long, Gen, Rose, Sakura, and Dan. Start up another Arcade Mode, get 2 or more Perfect Rounds and get 2 Ultra Finishers. Beat Akuma when he challenges you (after the Seth battle). You cannot use continues.
Unlock Gouken: You must have all the unlockable characters unlocked that are available up to this point, including Akuma. Restart Arcade Mode and get 3 Ultra Combo Finishes and 2 Perfect Rounds. Gouken will challenge you after the battle with Seth. Win the match to unlock him. You cannot use continues.
Unlock Seth: Clear arcade mode with all the characters (including Akuma and Gouken). If you managed to unlock Seth with different criteria, it might be a bug (but the unlock is still valid for your profile).

By the way if you play the survival challenges you might fight against them anyway I don't have Akuma but I fought him in survival mode... and lost (I didn't have much health left)