The Shadow People
As chilling as they are mysterious, the Shadow People have existed and made themselves known throughout history - in games, movies, tv shows, and even in certain religions/mythology. Their true nature - whether they're malevolent beings or supernatural guardians - is an often confusing aspect, though it could just be as simple as that: they're as varied as humankind. The Shadow People could, in fact, be an otherworldly reflection of humanity; creatures that think/have the same type of society as humans...albeit in a more anomalous reality. In fact, their hazy nature, and a lack of clear details on their origins, go beyond them being called monsters or humans. In truth, they are what's known as an "anomaly."
Nobody really knows what the Shadow People's world looks like, considering the fact that no one can travel dimensions like they can. However, if the 'Dark World' trope is to be believed, the world of the Shadow People could just be another version of Earth, with obvious variations here and there. For the sake of names, however, let's assume there's nothing but darkness in their world. We'll call their home, "The Void." The Void is a dimension that exists outside of time; a place where there is only infinite space in every conceivable direction.
Rarely, a shadow will "solidify" itself for the sake of being visible to the naked eye. Even more rare is when they try to actually talk to people.
The true appearance of the Shadow People themselves is nigh-impossible to pinpoint - being made of next to nothing but darkness - but the most common accounts describe them as mostly humanoid in shape. Though, there have also been times where a "shadowy animal" was sighted, meaning that there are also 'shadow versions' of wildlife. One defining feature that stands out, however, is that the Shadow People have various 'eye colors'; possibly the only part about them that even remotely has color. The more 'passive' shadows are mostly seen with white eyes, whereas the more 'active'/aggressive shadows have red/green eyes, making it possible that their society is based on eye color. Also, while it's never told that the Shadow People can speak, they may indeed have their own language; but because they exist between dimensions, it's hard to hear what they're saying - almost like they whisper in an unintelligible language. Supposedly, there exists a mask of some sort which allows people to see and hear them when the Shadow People talk.
The Seven Gangs
For anyone who assumes that Shadow People live lives relevant to that of humans, you'd be right. However, most Shadow People take on the role of being "omens of death," which means some are tasked to watch over the dying, and ensure that the demons do not interfere, or try to steal the soul as it leaves the body. And because there are some who take their work of overseeing a person's fate more serious than others, the seven gangs of the Void were formed. The four "lower" gangs exhibit a calm nature, and bring comfort to those due for the next world. The three "upper" gangs however, are more aggressive and hateful toward humanity for the cruel and corrupt ways they treat each other, and take great pleasure in terrorizing them as punishment.
The seven gangs do not get along well with the FreeShades - seeing them as "inferior outcasts of the Void" - and even when they sign up with a contract to work alongside them, the gangs are determined to one-up the FreeShades in hopes of walking away with a bigger cut of the reward. To double their efforts, the gangs split up; the lower gangs head to the human settlements to investigate, while the upper gangs march on to the demon realms to get information on the main target, one way or another.