The "Mirror Folk", with individuals also known as Reflections or Mirror Images, was the name given by the Eighth Doctor to imperfect duplicates of humans that existed in a pocket of unreality that became connected to Victorian Edinburgh through the Silversmith's mirrors. (COMIC: The Silvering)
Created when a human looked at their reflection in one of the Silversmith's mirrors (which had become connected to a pocket of unreality through Spherion crystals), the Mirror Folk were able to step out of the mirror and into the “real world” so long as their original was trapped inside the pocket of unreality. Unlike their captured originals, the freed Mirror Folk would also not visibly age. If the original was freed, however, the corresponding Mirror Image would either be sucked back into the Mirror, if it was standing close enough, or simply dissipate into nothingness.
Though they shared their original's memory, a member of the Mirror Folk had a distinct, colder personality from the original, from whom they could also be told apart by their lack of a reflection of their own when passing in front of a mundane mirror.
When only part of the onlooker had been visible in the reflected image, only the part of their body that had been in frame materialized inside the pocket of unreality. This led to the creation of the “widowed reflection”, an army of grotesque half-beings assembled by the Mirror Silversmith as his minions. (COMIC: The Silvering)
The first of the Mirror Folk to come into existence was the Mirror Silversmith, the mirror image of the Silversmith himself, the human magician who had unwittingly built the Silversmith's mirrors that allowed the connection of the pocket of unreality to Earth. After capturing his original and clasping him in irons inside the pocket of unreality, the Mirror Silversmith worked to create more Mirror Folk, harvesting the psychic energy of the captured humans to stabilize the pocket of unreality into a city, “Mirror Edinburgh”.
After many years during which he amassed his people, however, the Mirror Silversmith crossed paths with the Eighth Doctor and Josie Day, who managed to free the human prisoners from Mirror Edinburgh, unmaking all Mirror Folk but the Mirror Silversmith. The true Silversmith was finally carried out of the pocket world by the Doctor, sucking the Mirror Silversmith back into the world from which he had sprung even as it collapsed around him. The Doctor then used his sonic screwdriver to shatter the Mirrors, seemingly destroying what remained of the pocket universe, and the Mirror Silversmith with it. (COMIC: The Silvering)