Are Ghost Pokemon Undead? Edit
Many people think Ghost Pokemon are actual ghosts. There are other theories, however. Some say they're unique Pokemon, and that they feed on the leftover aura after a Pokemon (or person) dies. Others suggest that they get the experiences of those who have passed, at times taking on the memories of the deceased as their own. This theory would explain why ghost Pokemon seem to have their moods and even personalities dictated by what happened to the deceased to cause their deaths.
The Silph Scope and Invisibility Edit
You need a Silph scope to see ghost Pokemon. Sometimes.
Gastly, Haunter, and Gengar seek out places of death. They acclimatize to those regions like sugar dissolving in water, becoming an invisible part of them. The Silph scope is able to detect ghost Pokemon and is necessary to find them in places they've acclimatized to like this. But if they're captured and taken to other regions, they haven't acclimatized and the Silph scope is no longer necessary.
Humans are naturally normal type and are thus immune to possession. However, note that the psychic class changes the trainer's type to psychic, meaning they would be vulnerable to ghost-type attacks. While we would advise against incorporating possession into your game, it is an option that we felt should be mentioned.