How does the "willing creature" aspect of the Protection from Evil and Good spell work with a creature that's already possessed?

Question

The following scenario happened in an old game; The DM already made a ruling on the issue, which is fine - but I was curious how the "willing creature" aspect of the protection from evil and good spell works if the creature is already possessed.

In that game, we encountered a possessed child. I tried to cast protection from evil and good on the child so that they could fight the possession by having advantage on any new saving throw against that effect. However, the spell says it must be cast on a "willing creature".

If they are already possessed, obviously the spirit won't agree to the protection from evil and good spell being cast on them. So how can an already possessed individual get advantage on saves against possession?

Answer 1

The child in your example could be considered "willing"

While "possessed" isn't a clearly defined condition in 5e, the Ghost statblock does include the Possession action, part of which says (Monster Manual, page 147; emphasis mine):

The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.

This description states two key things:

1. That the target is not deprived of awareness
2. That the ghost cannot be the target of a spell

At least in the example of a ghost possessing a victim, the victim remains aware, and it is the victim who is the target of a spell, not the ghost. Thus, if the child is willing to have the possessing spirit cast out, they would be the target of the protection from evil and good spell, and would gain advantage on any subsequent saving throws against the possession.

Answer 2

The "target" of the spell is the child, not the spirit. The description of the Protection from Evil and Good spell states:

If the target is already charmed, frightened, or possessed by such a creature, the target has advantage on any new saving throw against the relevant effect.

...which would make no sense if the ghost/entity possessing the child were the target. If the spell grants advantage to those who are already possessed, it proves that it can be cast on already possessed creatures.