Hollows take on a humanoid female form and are usually equipped with a staff similar to that of the Vessels. It is unknown if they are capable of human speech. Their bodies are somewhat transparent and look somewhat like spirits or ghosts, but their bodies are tangible yet hard to hit due to their high speed stat. Their legs usually must be bound in order to be hit. Most Hollows look alike, but their coloration varies between species. When encountered, Hollows usually attack in groups with each Hollow having a specific role in their party, similar to your guild. Their attacks are usually elemental ones instead of physical attacks.
At a few points in the game, they possess a Vessel's deceased corpse to lure the party into battle (which succeeds if they try to interact with the Vessel), abandoning the corpse and doing the fighting themselves if the ploy works.
Vessels and Hollows have been enemies for years due to the hostility Hollows show to the Vessels. The Hollows normally attack the Vessels' village in pursuit of the Medium for an unknown reason. It was stated that once, Hollows and Vessels lived together in harmony. Then, everything changed around the time the Medium was taken under the care of the Vessels. Then the Hollows defected.