There are four secret trophies in Dynasty Warriors 9, and here is how to get them:
Thronged with Visitors (Silver): Obtain 10 hideaways.
Unrivaled Warrior (Silver): Raise an officer's Attack to 900.
Helter-Skelter (Bronze): Complete a mission within 1 minute.
Like a Bird (Bronze): Juggle an enemy in the air for 5 seconds.