He is the leader and founder of the Guardians. Though Manny never makes an actual appearance in the film, he seems to be a wise and benevolent immortal being. It is implied that he can predict the future.
During the 18th century, Manny revived a young man named Jackson Frost who had drowned while saving his younger sister. Jackson was reborn as the winter spirit, Jack Frost. Though Jack was frightened at first, the sight of the moon calmed him, he heard Manny's voice telling him his name.