Shadow people are spirits. This much is known. Some people theorize shadow people are time travelers, but unless all spirits are time travelers, this is not the case. Shadow people present on the Earth as black, humanoid apparitions, although they sometimes embody in black masses or black clouds. Shadow people are etheric in substance, just as all spirits are; therefore, it makes good sense for one to believe that shadow people are spirits no less.
As spirits, shadow people are either ghosts, which would be souls, demons, which would be angelic, or their own entity, which could be anything. Shadow people in the historical record indicates a sacred connection to humanity. Earliest mentions of shadow people date back to ancient Greece when entities known as shades were first described. Shades were considered souls in ancient Greece. A little down the road, both in time and place, the Quran made 29 references to shadow people––referencing the entity by the name jinn. Jinn are said to be entities in a parallel universe who can be equatable to demons. For this reason, shadow people are demons has a good backing in religion. However, Christian interpretations sometimes regard shadow people as souls. The fact of the matter is, no one knows. I believe that they are souls. Shadow people are a common feature in schizophrenia. In my system of belief, schizophrenia is considered a soul enterprise; schizophrenia is considered a punishment for bad karma in a previous life; for example, a murderer in one lifetime might reincarnate as a schizophrenic, in my system of belief, if he or she does not fix his or her Karmic imbalance in Heaven before reincarnation commences. For this reason, because I doubt demons are enlisted for any long term service in assisting souls, I conclude that shadow people are souls. But I could be wrong––they could be demons.