Tsar Lunar, known as the Man in the Moon or simply "MiM", are the names that was used and adopted my a Wioni who lived on the moon and protected the dreams of all children. Unlike the rest of his species, MiM was one of the few Wionis who have empathy. Although he never reveals his rule name, he did, in his own way, told the Guardians of his and their true nature.
Although he never actually speaks, he can communicate through the shining of the moon's light, and is frequently spoken to by other characters. Pitch considers him an "old friend", North refers to him as "Manny", and Jack often asks him who he (Jack) is, and why he was chosen.