Whether the term “Friendly Ghost” is supposed to be a joke, or it’s just an exaggeration, it’s certainly not entirely accurate. Casper these creatures are not. But, they can tolerate a small number of people. A handful, or a ‘party’ if you will.
As ghosts, they cannot physically interact with their environment. They cannot lift objects, or even brush dust off of objects. They cannot be restricted by walls, no matter how thick. They cannot stab anyone, and they cannot be stabbed.
Friendly ghosts can be harmed by magic or magical weapons, but cannot wield magical weapons themselves.
They advance as elves for experience and saves, and have a 1d4 hit die. They move and fly at the same speeds a human can walk or run.
Friendly Ghosts have a 5-in-6 stealth chance. It’s easy to move undetected when your body passes through objects rather than knocking them over. But their partially transparent bodies are still highly reflective, which prevents them from being completely undetectable.
Friendly ghosts should be treated as 2 levels higher than normal when determining an enemy’s stance towards the party, or when making a reaction roll to intimidate or frighten an opponent. They can also serve as a bridge between their living friends and other creatures from beyond the grave. Friendly ghosts are able to parley even with unintelligent undead, such as zombies. And with any undead creature, the Friendly Ghost receives a +2 to their reaction rolls.
Players who don’t wish to roll a new character can instead opt to turn their dead PC into a Friendly Ghost. The character loses all of their class abilities, but retains whatever level of experience they managed to achieve in life. (i.e. George the level 3 fighter can become George the level 3 Friendly Ghost).