The Ariels are the Hidden People of the sky-country. They are city dwellers and live in the huge, beautiful cloud-city of Seraphimia. This city looks rather like a Venice in the sky, set on a sky-lagoon, and with lots of cloud-canals down which go gondola-like boats.
On one side of Seraphimia, the lagoon leads into the sky-ocean and then to a weird cloud-valley where float all kinds of strange shadows, leftover shapes and bits and pieces from the beginning of time. This valley is also the way into the land of the Ariels. Nobody lives there.
On the other side of Seraphimia is the wild wasteland country where dwells beings like the storm giant, Uryon, also known as the Flying Huntsman because he gets around on a huge black hunting horse with a pack of red-eyed dogs. Uryon isn't an Ariel; he's older than them, he's from the beginning of time.
The Ariels can be tall, small, thin or large; there are all kinds of people amongst them, good, bad and in-between. Generally speaking, they're friendly to the other Hidden Worlders--but that's not always the case, as Thomas and his friends find out to their cost in Book 5(which will be out towards the end of the year). The Ariels are famous for their baking--they make the best cakes in all the Hidden World--for their knowledge of stars, and star-magic, and of course, flying!