Ghost HP:

On top of their [Physical HP], each character also has a [Ghost HP]. This represents the number and strength of the ghosts that are currently possessing the user.

This encompasses two stats, [Maximum Ghost HP] and [Current Ghost HP].

[Maximum Ghost HP] is the number and strength of the ghosts currently bonded to the user. This number is what your [Current Ghost HP] will increase to whenever you cast the "Gather Ghosts" spell. The idea behind this is that even when ghosts are sent to perform spells, they always will have a home in your body. This number can increase or decrease in several ways:

If you are the target of a ghost's "Possession" attack, and fail or forgo a resistance roll, you will automatically have your [Maximum Ghost HP] increased by the amount dictated by the "Possession" spell. This will also increase your [Current Ghost HP].

You can willingly perform a ritual to be possessed by more ghosts. This requires a puzzle weapon containing the appropriate amount of ghost HP. This will also increase your [Current Ghost HP].

If you are hit by a puzzle weapon, your [Maximum Ghost HP] is decreased by the amount of puzzle damage inflicted by the puzzle weapon. This damage cannot exceed the storage capacity of the puzzle weapon or your [Current Ghost HP]. This will also reduce your [Current Ghost HP]. Said weapons can be used in later rituals, but the same rules apply.

[Current Ghost HP] is the number and strength of the ghosts currently contained within your body. This number fluctuates much faster than maximum ghost HP, as it is used to fuel spells. Using the "Gather Ghost" spell will increase your [Current Ghost HP] to the value of your [Maximum Ghost HP]. On top of the changes caused when the [Maximum Ghost HP] changes, this number can increase or decrease in several ways:

When you are hit with a glass weapon (or other dispersing weapon), your [Current Ghost HP] will be decreased by the amount of dispersal damage from the weapon. [Current Ghost HP] can only be reduced to 0 in this manner.