Hagoromo Ōtsutsuki

The was the legendary figure who founded the arts of ninjutsu and created the ninja world. He defeated the Ten-Tails and sealed it within his own body, thus becoming the first jinchūriki.

Background
According to Jiraiya, the Sage was a monk who appeared long ago in an age of endless war. He was the first to discover the truth of chakra, and with it he tried to lead the world to an era of peace instead of conflict. He travelled across the land, spreading his ideal and religion, the, which eventually came to be known as ninjutsu. His great deeds and ultimate desire to bring peace to the war-torn world made him widely known as the.

Centuries ago, the sage defeated the most powerful demon in the ninja world, the Ten-Tails, and was praised as a god for the virtue of his victory. In order to protect the world from the Ten-Tails, the Sage developed a certain technique to seal the beast within his own body, giving him the ability to harness and control its overwhelming power; he became the first jinchūriki. However, this offered only a temporary solution. In the event of his death, the seal would break and the Ten-Tails would once again be unleashed upon the world.

Aware of his impending fate, the Sage used to divide the Ten-Tails' chakra into nine individual constructs that would become known as the tailed beasts. As a final precaution, he used Chibaku Tensei to entomb the physical remains of the Ten-Tails body within a gigantic stone prison, which he hurled into the heavens, creating the moon. It is later revealed that the Sage gave names to these beasts. The Sage would leave behind a tablet for his descendants, detailing the history of his struggle with the beast. Only those who possess the Rinnegan can fully decipher the contents of the tablet, while a reader with a lesser dōjutsu can still partially interpret the information. Some time after creating the tailed beasts, the Sage told them that would be together even when separated, and that one day they would again be a single entity.

The Sage's ultimate wish was to establish peace across the world, but it was a goal that would never be achieved in a single lifetime. Knowing this, he chose to entrust his strength, dream and legacy to his two sons. The older son, who inherited the Sage's "eyes" (his powerful chakra and spiritual energy), believed that power was the true key to peace. The younger son, who inherited the Sage's "body" (his powerful will and physical energy), believed that love was the true key to peace. On his deathbed, the Sage chose his younger son to be his successor. Overcome by bitterness and envy, the older son attacked the younger, beginning a war between them. Their feud would continue on through their descendants: the Senju clan and the Uchiha clan.

Personality
The Sage of the Six Paths was a man who sought peace by spreading his wisdom across the world. He was exceptionally wise, as he was able to bring about a great change to the world by simply disseminating the principles of chakra and his religion. He also seemed to care a great deal for the tailed beasts when they were still with him; he gave them each a name as a sign of their individuality, and told them that no matter how far apart they grew, they would always be together, and would someday be led down the right path. In return, the tailed beasts showed respect and adoration for the Sage. Kurama even shed a tear at the thought of his imminent passing, suggesting that the beasts might even have viewed him as a sort of father-figure.

Appearance
Although initially depicted as a silhouette, several details about the Sage's appearance can be inferred. He had short hair that spiked up in the front with two distinctive pieces of hair resembling horns. He wore what appeared to be a cloak with a high collar, revealing his necklace with six red magatama. These symbols were later mimicked by Nagato's Deva Path, and the seal around the neck of Naruto's Nine-Tails Chakra Mode. On the back of his cloak was a stylised Rinnegan with nine magatama in three rows beneath it. In one appearance where his face was partially visible, he was in an elderly state.

Abilities
Though not a great deal is known about all the skills the Sage possessed, it's safe to assume that he was an immensely powerful individual. Even among today's standards, Kurama still considers him the world's most powerful individual, having been the only person to defeat the Ten-Tails, a being that was revered as a god, causing the Sage himself to be seen as a god. The Sage was also revered as the man to first understand the very nature of chakra itself. This wisdom, combined with his kekkei genkai, the Rinnegan, allowed the Sage to create the first modern ninjutsu. With such powers at his disposal, the Sage obtained a great following, as men and women from across the world strove to learn from his teachings. As the Sage's disciples grew continuously greater in number, it led to the creation of ninja clans, and later, ninja villages. It is impossible to overestimate the Sage's effect on the course of the ninja world's development. As the first true ninja, he created the ninja world, revolutionised it, and left it forever changed.

The Sage was shown carrying two objects. One was what appeared to be a large katana in his hand, whilst the other was a shakujō — a Buddhist ringed staff which is meant to be used in prayer, but can also be effectively employed as a weapon.

Rinnegan
The Sage was said to have been the only shinobi to have fully mastered the Rinnegan, allowing him to manipulate all five types of nature chakra. He is revered as one of the most legendary figures in the ninja world, and is most likely the strongest ninja that ever existed. He was so powerful, that he created the moon using Chibaku Tensei on his deathbed. Even Nagato stated that his power paled in comparison to that of the Sage's. As a wielder of the Rinnegan, the Sage also wielded all the abilities of the Six Paths Technique.

Jinchūriki
The Sage developed the first technique capable of sealing a tailed beast inside a human body, making himself the very first jinchūriki in history. Having the Ten-Tails' great power contained within him, the Sage developed immeasurable power and chakra that far surpassed all others. Even on his deathbed, the Sage was able to employ his Chibaku Tensei technique on a massive scale and use that chakra to create the moon and seal the body of the Ten-Tails within it, a clear testament to his incredible power.

Yin and Yang Chakra
By using the Yin chakra, that comes from the spiritual energy, and the Yang chakra, that comes from the physical energy, he had the ability to create form from nothingness with a mere thought and then bring it to life. The Sage created the nine tailed beasts from the Ten-Tails' chakra by using this ability.

Treasured Tools
The Sage wielded unique tools which became known as the Treasured Tools of the Sage of the Six Paths, each of which possess a unique and powerful ability, but all of which require a vast reservoir of chakra, which would quickly kill any normal human, in order to be used. Being the jinchūriki of the Ten-Tails, this posed no problem at all for the Sage. The five treasures are: the Bashōsen, Benihisago, Kōkinjō, Shichiseiken, and the Kohaku no Jōhei. By combining the Benihisago, Kōkinjō, and Shichiseiken, the Sage (or any user) can capture the word soul of his victims, and then draw them into the gourd and seal them away. The Bashōsen can generate any of the five basic elements. The Kohaku no Jōhei can seal anyone without the elaborate procedures used for the Benihisago. The five treasures eventually fell into the possession of Kumogakure, and then into the hands of the Gold and Silver Brothers, who made use of the treasures in their criminal activities. Kumo however, managed to retrieve the Kohaku no Jōhei which was kept in the Raikage's office.

Legacy
As time passed, the Sage ultimately faded into legend. His bloodline divided into the descendants of his two children, which would eventually form the Senju clan — and through distant relations the Uzumaki clan — and the Uchiha clan. As a result of Madara Uchiha acquiring and integrating Senju DNA into his body, the Rinnegan would later resurface. Though he noted nothing happened initially after the DNA integration, when he neared the end of his natural lifespan, the dōjutsu finally awakened. With that, Madara Uchiha managed to undo the Sage's seal on the vessel of the Ten-Tails by summoning it from the moon and formulated the Eye of the Moon Plan to become the Ten-Tails' new jinchūriki.

Because Nagato, a pupil of Jiraiya's, manifested the Rinnegan, the latter suspected Nagato to be the reincarnation of the Sage himself. In reality, Madara secretly transplanted his own Rinnegan to the boy, without his knowledge. Nagato, as a descendant of the Senju-related Uzumaki clan, was the perfect host for the dōjutsu; however, Nagato noted that his own power would have paled in comparison to the Sage's. Others, including Kakashi Hatake, noted that the Rinnegan was viewed merely as a mutation. Kurama and Son Gokū's recognition of the Sage (and the tailed beasts' apparent status as the only survivors from the Sage's lifetime), lends credibility to his existence.

Before he died, the Sage told the young tailed beasts that someone would come and lead them on the right path. The tailed beasts would all later come to agree that this person is Naruto Uzumaki.

Trivia

 * The necklace that the Sage wore 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 Sage shares the same nickname with the Third Hokage, Hiruzen Sarutobi. They were both known as the.
 * Due to the Gold and Silver Brothers' ability to assimilate Kurama's chakra by eating some of its flesh, the Fourth Raikage suspected them to be distantly related to the Sage.

Quotes

 * (As quoted by Jiraiya)
 * (To the tailed beasts)