When Jin started seeing the creatures on the other side of the mirror, he was eight--too young to have closed his mind to this kind of thing, but old enough to know better than to talk to grownups about it. Being clever, he also found out that, among his peers, these visions made him unique.
Jin wasn't at all frightened, because the creatures didn't seem to notice him; even if they did, they paid him no mind. And so he spent hours in the bathroom, in front of his mother's vanity, or with the compact he'd stolen from his aunt, studying.
Some of them floated. Some crawled. Some seemed to walk on surfaces a few feet or inches above or below the floor. Some were completely alien in appearance, with bodies that made no sense. Some defied spatial physics. Some were made up of angles that didn't exist. Some were just indistinct. Some were horrifying. Some were even kind of cute. Overall, though, they just were.
As the weeks and months passed on, he began to group and classify them by their general characteristics and demeanor. It became apparent as he did so that many of these families, genii, and species were just as detached from the inscrutable business of other families, genii, and species as he was from theirs. After a great deal of consideration, he concluded that what lay before his eyes were layers upon layers of different planes of existence.
Therefore, at a shockingly early age, Jin understood his insignificance in the larger fabric of the universes. And, for the most part, he was okay with it.
Time and this knowledge swung him back and forth from empty nihilism to boundless compassion; from boldness to ennui. He became, in effect, a teenager. All the while, the beings that walked, crawled, and floated above, below, and through him became a part of his boring routine, in the mornings when he tried on clothes, and at night, when he brushed his teeth. They lived together indifferently, like neighbors in a tenement building.
One evening, all of that changed.
It happened as he was relaxing in his bedroom, dealing with a dull homework assignment. A mirror hung from the door of his bedroom closet, and he faced it, as he always did, finding comfort in the comings and goings of his invisible companions. An aspiring musician, he caught wind of a passing muse and began making up a little tune to hum.
And that's when the beast paused. It was one of the hideous ones--the kind other children imagined living in their closets. This one wouldn't fit, though. It was as tall as a streetlight and as wide as a bear. Jin thought nothing of its actions, until it turned what was probably its head and looked straight at him. He continued to hum--a little bit nervously, though--while trying to put aside any sort of concern. In the past nine years, there was no evidence to suggest that they were aware of him in any way whatsoever. Even when it began to lumber in his direction, he assumed it was only a coincidence. He went quiet, though, just in case.
But when it halted right behind him, he started to get concerned. And when it deliberately ducked down to look through the framed pane of glass on his door, terror set in. The beast groped blindly for him, more out of curiosity than malice; but the effect was the same. Where its paw raked across his skin, a fiery chill tore through and beneath his skin.
The beast recoiled. It glanced this way and that, and it bounded off. All of the other creatures in all of the dimensions all around him scattered like startled fish. For a moment, and for the first time in nearly a decade, Jin was alone. Gradually, they drifted back, and things returned to normal.
Except that things weren't normal anymore.
It took him a few minutes to catch his breath and compose himself. After he did, Jin walked over to the mirror, gazed deep into the reflection of the worlds around him, and hummed.
This is a bit of world-building for a fantasy novel my wife and I are writing.