Spirit Mediums (also known as Espers) can see all ghosts. Ghosts can be seen by only five people of their choice. If they wanted anyone else to see them they'll need a Spirit Medium to use a spell to temporarily allow them to be seen by whomever they want.
Ghosts are the result of people who are waiting to be accepted into either Avalon or Tartarus (heaven and hell respectively, Limbo Ghosts). People also become ghosts when they have unfinished business on Mobius that the "Gods" deem important (Missionary Ghosts) or when they simply wish to reside and watch Mobius' adventures (Observer Gohst). People who choose to reside on Mobius can never enter Avalon or Tartarus. Divergently, people who go to Avalon and Tartarus can never come back as ghosts. While their soul is still on Mobius they can be brought back to life at least twice. When brought back their ghost form/soul will be jacked from whatever it's doing and automatically returned to the persons body.
Little side idea, there's another part of Avalon and Tartarus that allows for reincarnation.
Ghost have the ability of mind control and hypnosis, as well as to speak with Spirit Mediums. They are also transparent, can go through walls, disappear and fly (yes, they are much more unique than the other guys). They can cause stereotypical paranormal activity around them such as posses objects around them (only unconcious and inanimate objects). They can become visible/opaque in the dark (as well as at will for short bursts of time) and control extremely nerfed versions of their abilities when they were alive. They can also fade into a visible shadow.
In tradition to keep up with Sonic norm I think ghosts should be weak to light. It inhibits their abilities severely and makes them visible as well as touchable. Ghosts should also be incapable of speech
Ghosts reside in places that are important to them/the area of their death as that is where their powers thrive. They can go anywhere but the farther they go away from this place the weaker they become.
Missonary Ghosts have an entire year to do their objective. While limbo ghosts have one week while waiting for judgement.
What does you guys think? Is it good? Adress any complaints below! Thanks for reading!