The Jjaro (also spelled Jjarro) are a race of beings who somehow escaped the universe.  In their wake, they left all of their technology behind, and still managed to get involved in the affairs of various inhabitants of normal space/time. They are also occasionally referred to as Yrro, but it is unclear whether this refers to the Jjaro as a species or just to an individual member (or whether Jjaro is a species at all).
They are accepted as the creators of the S'pht, and as being involved in the creation or modification of the player-character. The Jjaro are almost definitely responsible for the player's time traveling antics in Marathon Infinity.
In Pathways Into Darkness, they warn the president of the United States of America of the impending awakening of a sleeping god in the Yucatan Peninsula.