The seraphim – sometimes referred to as angels – are immortal beings that wander the earth. They all have a different purpose to serve; some communicate with humans —prophets— and make sure that little things that need to happen happen, some of them watch over certain individuals, a few act as reapers, and so on. They don’t really have any personal agendas, and as far as they know they don’t really have a boss to answer to either, they just… do what they do. They’ve always been there, and they’re at peace with just carrying out their assigned tasks without questioning them.
They’re awkward creatures; rather chatty but not particularly bright. They exist on a different plane from humans, so most people can’t see them, and the seraphim can’t interact with these people either. Humans who can see and interact with the seraphim are typically prophets, whose purpose is to help the seraphim carry out tasks that require direct interference.
Samael is a prophet, so can hear the seraphim everywhere he goes. 24/7. He is not too happy about this; he overhears a lot of things he wishes he never knew about, and frankly the seraphim’s constant babbling is just annoying. Liv can hear the seraphim, but she can’t see them and she doesn’t understand what they say. To her, they just sound like a chime of bells. Milk sometimes catches a glimpse of the creatures out of the corner of their crystal eye, but they don't really pay any attention to the winged beings; the seraphim don’t seem to interest them much.
Demons are very powerful, complex, soulless beings from a dimension incredibly different from the one we're used to. One can summon them into our dimension, and while they're here they have access to both the physical plane and the spiritual plane — but as long as they're in our dimension they can’t only exist on the spiritual plane: they’re required to take on a physical form. The form can be anything they choose, and they can change it at any time. Damage to their physical form does not damage the demon itself, but they can take damage from supernatural or magic attacks.
Summoning a demon is very risky business; they typically have no desire to be here, and are generally very unhappy about being violently ripped from their own comfortable dimension into ours, which is why you need a sigil to bind the demon. The summoner and the demon then enter a contract stating the circumstances of the demon's stay here, and the conditions that will grant the demon its release from this place.
Needless to say, while demons make extremely powerful slaves, they’re hostile, involuntary and antagonistic workers. Even if you manage to not mess up the the summoning part, then there’s the business of forming a contract with a pissed off, powerful and capricious being that has no desire to help you (or to even be here on this plane of existence) and it'll most likely attempt to kill you at the first chance. That being said, there are exceptions; there’s a few demons who don’t mind this dimension as much, and as long as their summoner shows a hell of a lot of respect, and presuming they can negotiate the conditions of the contract, they might not mind cooperating. However, in this day and age, summoning demons is not a very common practice.
Upyres are vampire-like creatures that don’t explicitly require blood, but they do have a profound appetite for human flesh. Sunlight doesn’t kill them, but they’re very sensitive to it and experience great discomfort when exposed to bright lights. They have a very long lifespan. Upyr societies are located deep in the woods and are avoided by most outsiders, but there are merchants of other species who deal with them at times.
There’s a kind of unspoken rule that the upyres and the humans keep to themselves respectively, and any human stupid enough to wander into upyr territory for any other reason than pre-agreed upon business is generally considered fair game. Therefore, if such a person ends up becoming a upyr’s happy-meal, other humans will agree that they had it coming, and won’t make a fuss about it. Likewise, if a upyr decided to start munching on people in an area claimed by humans, humans would promptly hunt it down and kill it, and the upyr society would not retaliate.
Nightwalkers are forest-dwelling creatures with a very close relationship to nature, and an affinity for mud. They live in mounds, and feed mostly on small animals like birds and rabbits. They don’t mind eating larger ones, such as lost humans – it's just that those tend to be more of a hassle to catch. Nightwalkers can see spirits, as they have free access to the spiritual plane. Gender plays virtually no role in nightwalker society, instead individuals are referred to by titles assigned to different trades and social statuses.