A hero in Grayplane is hidden and cannot be seen by enemies.
Attacks made by hidden characters ignore armor.
Hidden characters are revealed if they attack or take most actions, including nonviolent actions like Extinguish, but opening doors does not reveal them. Hidden characters are revealed if an enemy movement would pass through the hidden character's tile -- the enemy's movement is stopped. Hidden characters can also be revealed by the Search ability.
A hidden character can still be damaged by area-of-effect abilities that target the hidden character's tile.