Changeling

In the past, gateways to other worlds started opening in different parts of the human world. Before any other beings managed to warn humans, Fae showed up first and invited human ambassadors to visit their realm. The humans agreed.
There are rules to Fae and their world. Rules that should never be broken. The Fae, of course, never explain them to their guests. And so, the ones who broke them disappeared. Fae were warned that such an act could spark a war. In response, they released all the prisoners. Or, at least, things that looked like them. The changelings.

Puppets of nature

Changelings are a magical wonder in the field of the creation of artificial life. They can eat, sleep and dream, think for themselves (or rather, the way the original would), and in a special case (if their original has died), they can even have children. The kids born of such a relationship will always be of the species of the other parent.
The birth of a changeling starts when a Fae creates a poppet. Using the silks coming from the insects of their lands, a Fae creates the skin. The skin is then filled with sticks, leaves, and stones. When it is decently stuffed, it's time to sew it together. Then, a pinch of Glamour and an idea of who they will be mimicking, taken from the source, adding some life to it all, and a new changeling is ready.

Shaped by thoughts

Changelings can't really exist on their own. Even with Fae magic, it's almost impossible to copy the brain of a living, sentient, and sapient being, with all of its memories and wants. Because of that, Fae tend to take a few shortcuts. Instead of trying to replicate what's impossible, they let the nature of the changeling be shaped by the people around it, their feelings and memories related to the original.
In practice, changeling will be the person, the people remember original to be. If someone always considered the original to be smart, they will be smart. With a small number of people around the changeling, that would create a very simple imitation, but the more acquaintances of the original there are, the more fleshed out the impostor will be.
This shortcut leads to a certain downside. It tends to create feedback loops over a long period of time. Using the example from before, a changeling that imitates someone considered smart will act smarter and smarter. This will make more people think they are smart, which will further fuel the Fae magic. This makes changelings effective as spies only for about a year since a longer period will always make them draw attention to themselves in some way.

Servants of Fae

While changelings can live for years without knowing what they are and considering themselves normal, their Fae creators can always show up next to one of their creations. Once they do that, a changeling in their presence becomes a puppet they can control however they want. When no one else is present, Fae won't bother with moving them around. They can ask a changeling whatever they want and the construct will always respond with truth.

Changeling paranoia

When the Great Fae War was at its peak, the fear of being replaced by a changeling was at its highest. People started suspecting anyone who stepped out of the line. Anyone that seemed to be getting "too kind", "too mean", "too ambitious" and many other "too"s could be the next target of a scared and angry crowd. Soon, the lynchings started. It is said that for every 9 changelings killed, there was at least 1 innocent person that died.
With years, more technologies detecting the changelings were created, and the number of innocent people killed started dropping, however, the fear of someone close being replaced was still as big as before. Today, while the fear is still there, there are enough methods of detection and groups specializing in finding the missing people, that paranoia turned into mild concern. Once in a while, one can hear conspiracy theorists calling someone important a changeling. Such claim is often quickly proven wrong by the one accused, ridiculing the theorist.

Appearance

While their Glamour works perfectly, changelings mimic the appearance of the creature they are copying up to the smallest detail.
If something happens to the Glamour, their real form is revealed. In that case, they look like scarecrows made out of silk sacks, leaves, sticks, rocks, moss, and other things found in nature.

"Mirror, mirror, on the wall" - when the mask slips

Most of the changelings don't know their own true nature. Most of the things that will reveal what they are will show up only when the original is in the same realm they are. Those slips of Glamour are:

• Seeing their real reflection in the mirror.
• Bleeding leaves and sticks when hurt.
• Dreaming of original's current location.

When changelings learn what they really are and they sense that the original starts to close in, trying to reclaim their own life, most of them will have a hard decision to make. After all, they don't want to lose what they have. It doesn't matter that they are a copy, they also live that life. Some will turn aggressive, trying to kill the original. Some will try to hide or run away. Very few decide to talk with the original, hoping to reach a peaceful conclusion.

"I want to be a real boy"

There is a way for a changeling to become a real living being. If the changeling meets its original and if both of them agree to that, the original can accept the changeling as a living being separate from themselves. In that case, the changeling stops being their copy and won't be affected by the thoughts of other people. They become real people, with their own dreams, likes, and dislikes.
Many changelings who are given this option move out, trying to find a new life for themselves.
It is unknown whether this frees them from the influence of the Fae.