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.

Background
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.

According to Madara Uchiha, when the Sage was on his deathbed, he entrusted his power and dream to his two sons. The elder brother, born with the Sage's Rinnegan and spiritual energy, believed that power was the key to peace; the younger brother, born with the Sage's life force and physical energy, believed that love was the key to peace. In the end, the Sage chose the younger brother to be his successor, much to the resentment of his older son. Overcome by bitterness and envy, the Sage's older son attacked his brother, beginning a war between them. This eventually gave birth to the Senju clan, the Uchiha clan and the endless struggle between them.

Appearance
Although depicted only as a silhouette, several details can be inferred about the Sage's appearance. He had long, spiky hair flowing backwards, in a style similar to Madara Uchiha's. He wore what appeared 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 seemed to have horns on his forehead; this could be a forehead protector similar to the one worn by Jiraiya.

Part II
As of now, the only other person known to possess the Rinnegan was Nagato, also known as Pain, a shinobi from Amegakure and the former leader of Akatsuki. Whether this means that the Rinnegan is a unique Kekkei Genkai which isn't passed down by blood, or if Pain was 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, never referred to himself as the Sage's reincarnation, and, when using Chibaku Tensei against Naruto's six-tailed transformation, admitted that the Sage's mastery of the technique, and of the Rinnegan itself, far surpassed his own. When he is being questioned by Kakashi, Yamato and Naruto, Madara Uchiha reveals how the Uchiha Clan descended from one of two brothers to whom the Sage of the Six Paths banned from taking his place as the one to bring peace to the world.

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

Rinnegan
He was said to be the only shinobi to have mastered 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.

Trivia

 * Pain's Deva path wore a necklace very similar to the one worn by the Sage of the Six Paths, although Pain's was black while the Sage's was white. This might be a reference to the fact that they were similar, as they both had the Rinnegan, whilst at the same time very different, as Pain's twisted idea of peace threatened to destroy the world the Sage had 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

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