Every physical object has a reflection. And every soul has a reflection, in the Mirror Plane.
The Mirror Plane exists parallel to our physical one, and tends to ebb closer and farther to it on a regular basis. When the two dimensions are close to each-other, it’s possible for ethereal beings [a celestial, angel, or demon] and mortals in direct contact with them to cross from one to the other through a reflective surface- a mirror or a still body of water work best.
The Mirror Plane appears identical to its earthly counterpart, except devoid of colour and flipped horizontally- and, for the most part, empty. If you’re particularly inclined to spirits, you might be able to hear and see faint echoes of what’s happening on the other side.
After all, few make enough of a mark on the Mirror Plane to be more than an echo...
By being in contact with undisguised ethereal beings will increase the strength of a soul’s reflection in the Mirror Plane. If the echoes become strong enough, they’ll be able to take a physical form in the shape of a Mirror Demon. When a Mirror Demon is born, they appear as a mindless monochrome copy of their earthly counterpart, but as the echo becomes stronger with more contact, it becomes distorted and fragmented. The demon gains sapience, and their appearance shifts in ways they subconsciously desire. At the point where they gain sapience, a Demon will name themself. They’ll usually source this name from a mortal work of fiction. While some Mirrors are capable of speech, it's rare, and those who can talk are unintelligible to anyone other than other Mirrors. To anyone else, their speech sounds distorted and garbled, like this:
The anatomy of a Mirror Demon is comparable to an antique doll. Their skin has the texture and fragility of porcelain, and there's little underneath but a black substance. It's usually a soft solid, but begins to liquify when the skin above is damaged, sealing the wound but leaving lines where the cracks healed. This gives the appearance of the Mirror having "black blood" when in fact they have no blood of such to speak of.
Mirrors cannot be killed unless their skin is shattered completely, leaving the black substance to pool on the ground. Otherwise, they'll keep moving for as long as it's physically possible.
If a Mirror Demon and their mortal counterpart manage to come into physical contact, they’ll be bound to one another. They'll each have a vague hot or cold sense of where the other is, and if one is injured by something other than their counterpart, the other will feel pain in the same area, about an intensity equal to the severity of the wound. For especially severe wounds, a scar will appear at the site of the injury on the counterpart. Due to the fragility of a Mirror's body, these scars tend to manifest as cracks on their body, while they’ll simply be markings on the mortal.
After one member of a bound pair’s death from unnatural causes, whether it’s the Mirror or the mortal, the two spirits will merge into one.
If the Mirror is the one to die, the mortal’s body will be most likely overwhelmed by the influx of spirits. The stronger the Mirror was, the greater effect this will have on them.
They’ll at least convulse and faint, waking up with a severe headache and losing the last few hours of memories. At worst, they’ll be mostly comatose for some time, ranging from days to weeks to months. Even after waking up, they may be catonic or in a state of delirium for a shorter amount of time, and be physically weakened.
If it’s the mortal that dies, the Mirror themself will lose their lack of colour and most of their differences. Unless you knew the original very, very well, you’d probably think they were the real deal. For a few minutes, the fusing memories will confuse the Mirror into thinking they’ve always been the real one.
Wouldn’t that be a perfect opportunity for the Mirror to replace them, if they wanted to do so? Whilst Mirror Demons aren’t inherently evil, the Mirror Plane is a tedious place. A particularly malicious Mirror, bound to their counterpart, may find that life a much more interesting one to lead.