The Sage of the Six Paths (六道仙人, Rikudō Sennin) was said to be the legendary figure who founded the ninja world and was the first to invent modern ninjutsu. He was said to be the only shinobi to possess the Rinnegan, which presumably laid the groundwork for the other two great dojutsu, the Sharingan and the Byakugan. He is revered as one of the most legendary figures in the Naruto Universe, and is likely the strongest ninja that ever existed. He was so powerful that he was said to have created the moon using Chibaku Tensei.
As of now, the only other person known to possess the Rinnegan is Nagato, also known as Pain, an orphan from Amegakure. Whether this means that the Rinnegan is a unique Kekkei Genkai that isn't passed down by blood, or if Pain is directly related to the Sage of the Six Paths currently remains a mystery.