Spirit

Spirits are the the souls of deceased humans that remain on the physical plane and generally cause trouble.

Characteristics
Depending on the places where spirits gather they may or may not create a society. There are colonies where all the lonely ones gather, but also risky areas where it's survivor of the fittest. Most of them have some concept of gender, but without the corporal form there's not much point.

Spirits can eat, but it's not obligatory for them. Certain objects and living creatures leave traces of energy in the air called "Spiritium", which can be absorbed by the spirit.

Majority of spirits in the series have evil intentions, but there are some exceptions like the spirit family that was non-violent towards living people.