So Manny, sometimes referred to as Man in Moon or even just MiM, is the God figure in the Rise of the Guardians universe. Just to be clear, I have only read the first half of North’s book in the Guardians of Childhood series, so while some of what I say will come from that, most of this is based on the movie, since that is what I know.
Manny became the very first Guardian many years ago, around the time the Earth got a moon. It is Manny who chooses who to make into spirits for children to believe in and it is Manny who chooses which of those spirits will become Guardians. He watches over the children of the world through both the Guardians and his moonbeams, which act to him as angels might to God.