Faeries not only do not like to be seen, they do not like to have their names known or spoken.
Faeries can be full of nasty tricks.
In Victorian times, it was believed that faeries lived at the bottom of the garden. Faeries were always thought eager to possess human children, and so it was believed that they would steal an unchristened, unprotected newborn, and leave a changeling in its place. Sometimes the changeling was a piece of wood, carved to resemble a child and cast with glamour to have a temporary appearance of life. Sometimes it was a sickly faerie babe, or an old and withered faerie. Children thought to be changelings were often beaten, left exposed on a faerie hill, burned or otherwise tormented in an attempt to induce the faerie parents to change it back again.