Tsar Lunar, known as the Man in the Moon or simply "MiM", was the very first Guardian who lived on the moon and protected the dreams of all children his ship. In the movie, he is shown to be a silent, but active, observer of the transpiring events. He is the one who convinces the four Guardians that Pitch's return was a true threat, and the one who chooses Jack Frost to be the newest Guardian to aid them.
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. He is only ever represented in the movie as the moon itself.
In the book
The Man in the Moon (MiM) began his life in the Golden Age, when all dreams were possible. But when his family is attacked by Pitch, The Nightmare King, MiM finds himself alone on a small moon, revolving around the planet called Earth. MiM keeps watch over the children of Earth, and vows never to let their dreams be invaded by Pitch's nightmares. MiM calls to his side a band of heroes to serve as The Guardians.