Dragons are beings that can form a Resonance with humans. They have an appearance similar to the dragons of human legends, and thus were named as such.
The original Dragons were born from Thanatos and maintain a physic link to it. The vast majority of the Dragons on Earth, however, were created by humans so that they can be used in the war against Thanatos.
Dragons can change there appearance to their Communicator forms, which are humanoid in appearance.
In their original forms, Dragons are able to display their own unique abilities.