Spirit

Spirits are the restless souls of deceased humans that remain on the physical plane instead of moving on.

Characteristics
Spirits can take on a variety of forms, from small animal-like critters to relatively normal-looking humans to massive beasts. Most of them have some concept of gender, but without the corporal form there's not much point.

They often haunt areas in which they died or which they had an emotional connection to. Depending on the places where spirits gather they may or may not create a society. There are colonies where spirits gather to band together, but also more dangerous areas where it's survival of the fittest. The majority of spirits in the series have evil intentions, staying on the mortal plane only to cause trouble, but there are some exceptions that are non-violent towards living people.

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. They can also eat each other in order to steal energy from one another.

Powers & Abilities
All spirits are capable of typical ghostly feats such as levitating and passing through solids, and even haunting the dreams of the living. Spirits also often possess supernatural abilities similar to psychic powers. Some spirits have even perfected their own special attacks to use in combat.

Their main shared weakness is psychic power. A skilled esper can do battle with spirits and eventually exorcise them, which in the world of Mob Psycho 100 is effectively "killing" the spirit again, though this time permanently. Mob could also reduce spirits to forms much smaller and weaker than their original forms, such as with Dimple and the Boss, instead of outright exorcising them.