Dream Master, or "DM", is the Dream Paladin. He was the ruler of the dream realm and appeared in one of Tites dreams to warn him of Azun. Azun was Tite's mentor at the time, giving him magical spells and such for his adventure. DM later appeared with Tite and Azun after his dream and made sure to stick by Tites side.
---------------------------------------
*Introduction to "Full Sync"
In the comic, there is a special thing called Full Sync that all paladins have. This enhances their magic or evolves their magic as a whole. Once they reach this strength, a diamond appears on them to symbolize they are in Full Sync. Their clothing changes as well to show the evolution of their power. An example of a Full Sync would be DM's Dream magic. His Dream magic evolves into Dream+Nightmare Magic, showing the dark and light side of the dreams. Tites magic evolves from Steam into Smoke, becoming darker and stronger. Full Sync is a great utility if you are in a tight situation.