Alternate costumes
How to unlock alternate costumes for Ryu, Ayane & Momiji. You can only obtain Crystal Skulls on Normal or higher difficulty, also you only need to pick up the Crystal Skull to unlock Ryu & Ayane's 4th costume.
Momiji 3rd costume: Complete 25 Ninja Trials with Momiji
Momiji 4th costume: Complete 50 Ninja Trials total with Momiji
Ryu & Ayane 3rd costume: Collect all 50 scarabs
Ryu & Ayane 4th costume: Collect all 10 Crystal Skulls