Hagoromo Otsutsuki, or the Sage of Six paths (Rikudo Sennin), was responsible for the birth of the concept of ninjas in the Naruto universe.
Son of the terrifying Kaguya Otsutsuki, who came to the Earth to harvest the chakra of humans and reap the chakra fruit, along with Isshiki Otsutsuki. However, she changed her mind after she fell in love with King Tenji.
Hagoromo and Hamura Otsutsuki were the two children of Kaguya, as is revealed in Naruto Shippuden. While Hamura sealed himself on the Moon, Hagoromo Otsutsuki stayed on Earth to preach the concept of Ninshu, or Ninjutsu, and was often revered as the Savior of this World (Kono Yo no Kyūseishu).
Hagoromo Otsutsuki, or the Sage of Six Paths, was the first to possess the Rinnegan instead of the other Otsutsuki, who mainly possess the Byakugan.
Why Hagoromo Otsutsuki, or the Sage of Six Paths, is considered legendary in the Naruto universe
Although she had given up the quest of harvesting chakra, Kaguya got back on the bandwagon after tasting the chakra fruit, which appeared once in every millennium on the Divine Tree. Gaining an insane amount of power, Kaguya put an end to all the contemporary wars instantly.
However, she grew greedy of chakra and raised the Ten-Tails from the Divine Tree to perform the Infinite Tsukuyomi. But her plans were foiled by Hagoromo Otsutsuki and Hamura, as the two brothers sealed their mother into the moon.
While Hagoromo sealed the Ten-Tails within himself, Hamura stayed on the moon. Later, the Sage of Six Paths used the Creation of All Things jutsu to break the Ten-Tails into nine Tailed Beasts, giving each of them a new identity.
The rivalry that spanned ages
The two children of Hagoromo Otsutsuki and Black Zetsu's manipulation gave rise to the entire story of Naruto. Although the Sage of Six Paths just wanted to teach self-defense to his followers, the innovation of his elder son, Indra, changed the course of Ninshu completely.
As opposed to his brother Ashura, Indra was a prodigy and had discovered the concept of jutsu as a child. However, his view of the future of the Ninshu creed was quite different from that of his father and even Ashura. As a result, the latter earned the most praise from Hagoromo Otsutsuki, although Ashura's talent was nowhere near that of Indra.
This resulted in a rivalry that spanned over ages, ultimately carrying forward into the Senju vs Uchiha rivalry, with the incarnation of Indra (Madara Uchiha) and Ashura (Hashirama Senju).
In the following age, the rivalry trickled down to the next generation of incarnations, Sasuke Uchiha and Naruto Uzumaki. Fortunately, they could find a more peaceful resolution, just as Hagoromo Otsutsuki, the Sage of Six Paths, had foreseen.