Complete Asia Chapter with Kyo at level 10. Then highlight Chris at the character selection and press L1 + L2 + R1 + R2. All characters, including Orochi, will be selectable.
Orochi
Hold L1 + L2 + R1 + R2 and select Chris at the character selection screen.
Orochi Iori
Complete all chapters. Then hold L2 + R2 and select Iori at the character selection screen.
Orochi Team
Complete all chapters. Then hold L2 + R2 and select all New Face Team members at the character selection screen.
Saishu Kusanagi
Complete all chapters. Then hold L1 and select Kyo at the character selection screen.
Sendo Siblings
Complete Disease Chapter. Then hold L2 + R2 and select any Art of Fighting team member at the character selection screen.
Souiji Kusangai
Complete Souji and Aoi Chapters. Then hold L1 and select Kyo at the character selection screen.
