Somebody mentioned ghosts? 
I'm an atheist, but there's a part of me that is still open to accepting the existence of supernatural stuff since there's just so much out there that we still don't know about. Overall, I see ghosts as having similar personalities to when they were alive, so I see them as mostly being friendly/positive. There's an empty house on my street that's in really bad condition (I used it as a loose reference for the mayor's house in my comic), but whenever I walk by it, I feel a sense of warmth and nostalgia, so I think if there are ghosts there, they are happy ghosts with good memories of the house.
In my comic, I didn't have a plan or stick to a specific tradition with the ghosts, so I just portrayed them as I see them/as it is convenient to the plot. Most are little energy blobs with no memories of their past lives, but some have lingering ties/unfinished business and remember some things (these look like humans). The ones that are overcome with negative emotions in life become evil spirits, and become more powerful/negative the longer they remain undealt with. There's another type that hasn't appeared yet where the ghost decays/loses its memories to the point of just being a strong emotion or intention bound to a place.
