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.
Young Athena
Enter survival mode. Defeat the opponents until you face Athena. Defeat her to unlock her as a selectable character in all modes. To choose Athena, highlight Genjuro, then hold R and press X, Y, A or B. Athena is one of the best fighters in the game.
Zero
Hold Start and press Up, Right(2), Left(2), Down, Up(3), then any button at the title screen.