Here is the full list of all 12 Cynthia: Hidden in the Moonshadow achievements worth 1,000 gamerscore.
Reach the Tesana Valley.
Run away from the guard.
Pet a cat.
Open all chests with the alternative outfits.
Shoot down 20 training targets.
Put on one of the alternative outfits.
Unlock second type of arrows.
Unlock third type of arrows.
Solve the first puzzle with moon icons.
Finish the main story.
Fall into the trap.
Get through muddy terrains.