Hagoromo Ōtsutsuki

The 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 Eye Techniques, the Sharingan and the Byakugan. He is revered as one of the most legendary people 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. It is implied by Nagato, but not confirmed, that he did so using Chibaku Tensei.

As of now, the only other person to be known to possess the Rinnegan is Nagato, also known as Pain, an orphan from the Land of Rain. 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.

Appearance
Although only seen as a silhouette, several things can be made up of the Sage's appearance. He had long, spiky hair flowing backwards, similar to Madara Uchiha's. He wore what appears to be a cloak with a high open collar or armor similar to that of Hashirama, showing his necklace with six Magatama. He seems to have horns on his forehead; this could be a Forehead protector similar to Jiraiya's.

He is seen carrying two objects. One is what appears to be a sword on his back, the other is a Shakujō, a Buddhist ringed staff that is used in prayer, but can also be used to fight.

Trivia

 * Pain's Deva Path wears a necklace very similar to the one worn by the Sage of the Six Paths, but Pain's is black while the Sage's is white. This may be a reference to the fact that they are similar, as they both have the Rinnegan, and at the same time different, as Pain's twisted idea of peace is dangerous to the world. How Pain got that necklace has not been revealed.
 * The necklace of the sage of the Six Paths might be a reference to the Yasakani Jewel; a legendary magatama necklace that is part of the Japanese Imperial Regalia, together with the Kusanagi Sword and the Yata Mirror. The Yasakani Jewel is the only one of these treasures yet to make an official appearance in the series.