Hagoromo Ōtsutsuki

The was said to be the legendary figure who founded the ninja world and was the creator and first user of modern ninjutsu, essentially making him the first shinobi, in the modern context of the term. According to Jiraiya, the Sage was a priest who appeared long ago in an age of endless war. He was the first to understand the true meaning of chakra, and tried to use his power and wisdom to lead the world to peace instead of war. The Sage travelled throughout the world spreading his ideas and religion, which eventually became the techniques used by ninja of the modern era. 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, a shinobi from Amegakure and leader of Akatsuki. Whether this means that the Rinnegan is a unique Kekkei Genkai that isn't passed down by blood, or if Pain is directly descended from 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 era. Nagato, however, has never referred to himself as the Sage's reincarnation, and when using Chibaku Tensei against Naruto's six-tailed transformation admits that the Sage's mastery of the technique, and of the Rinnegan itself, far surpassed his own.

Appearance
Although depicted only as a silhouette, several details can be inferred about 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 worn by Hashirama Senju, showing his necklace with six Magatama. He seems to have horns on his forehead; this could be a Forehead protector similar to the one worn by Jiraiya.

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

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 very different, as Pain's twisted idea of peace threatens to destroy the world the Sage devoted his life to create.
 * 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
the sage appeared to have horns and a forehead protector
 * (As said by Jiraiya) "I am here to provide peace and order"