Hagoromo Ōtsutsuki

The was said to be the legendary figure who founded the ninja world and was the first to invent modern ninjutsu. Jiraiya said that he was a priest who in a time of endless war appeared, he was the first to understand the true meaning of chakra and tried to lead the world to peace instead of war. He travelled the world spreading his religion, which eventually became the shinobi technique's. 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, although Jiraiya, when he was alive, believed that the Sage could have somehow been "reborn" as Nagato in the present time.

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.
 * 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.

Quotes

 * (As said by Jiraiya) "I am here to provide peace and order"