Dan:
Hold Start and press Down, Left(7), then any button at the title screen.
Demitri:
Hold Start and press Right(3), Left, Up, Right, Down, Up, then any button at the title screen.
Evil Iori:
Hold Start and press Up(3), Down, Left, Right, Down, Left at the title screen.
Geese:
Hold Start and press Left(3), Down, Left, Up, Right, Up at the title screen.
Goenitz:
Hold Start and press Down(3), Right, Down, Left(3) at the title screen.
Mars Alien:
Hold Start and press Up, Down, Right(3), Up, Down, Left(2), Down, then any button at the title screen.
Random character:
Hold Back at the character selection screen.
Red Demon:
Enter survival mode. Defeat the opponents until you face the Red Demon. Defeat him to unlock him as a selectable character in all modes. To choose the Red Demon, highlight Tabasa, then hold R and press X, Y, A or B. The Red Demon is one of the best fighters in the game.
Shin Gouki/Super Akuma:
Highlight Balrog at the character selection screen then hold R.
Shin Mr.Karate:
Highlight Kasumi at the character selection screen then hold R.
Two Hidden Bosses:
Simply beat 35 fighters in survival to unlock Athena and beat 36 Fighters to unlock Red Arremer. Then hold the R button over Tessa to use Red Arremer and hold the R button over Genjyuro to use Athena.
Unlock Fighters:
Hold the R button over certain fighters to unlock:
Ryu unlocks Zero
Chun Li unlocks Demitai
Dhalsim unlocks Dan
Bison unlocks V. Ken
Vega unlocks Shin Akuma
Kyo unlocks Geese
Terry unlocks Mars People
Ryo unlocks Goenitz
Kim unlocks B. Iori
Katsumi unlocks E. Mr. Katate
Violent Ken:
Hold Start and press Up(2), Down, Left(2), Right, Left, Up, then any button at the title screen.