Here is the full list of all 12 Cynthia: Hidden in the Moonshadow achievements.
Run away from the guard.
Reach the Tesana Valley.
Pet a cat.
Open all chests with the alternative outfits.
Shoot down 20 training targets.
Put on one of the alternative outfits.
Finish the main story.
Unlock second type of arrows.
Unlock third type of arrows.
Solve the first puzzle with moon icons.
Fall into the trap.
Get through muddy terrains.