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.
Hidden characters
Alternately, you can access all the hidden characters except Red Demon and Young Athena on the character selection screen by holding R1 over the four top characters of the inner most rows of both sides. The following characters can be accessed by holding R1 while on the standard character's location at the selection screen:
Kyo: Geese
Terry: Mars People
Ryo: Goenitz
Kim: Evil Iori
Kasumi: Shin Mr. Karate
Ryu: Zero
Chun Li: Demitri
Dhalsim: DaN
M. Bison: Violent Ken
Vega: Shin Akuma
Mars Alien
Hold Start and press Up, Down, Right(3), Up, Down, Left(2), Down, then any button at the title 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 R1 and press Square, Triangle, X or Circle.
Shin Gouki/Super Akuma
Highlight Balrog at the character selection screen then hold R1.
Shin Mr.Karate
Highlight Kasumi at the character selection screen then hold R1.
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 R1 and press Square, Triangle, X or Circle.
Zero
Hold Start and press Up, Right(2), Left(2), Down, Up(3), then any button at the title screen.