The Fourth Hokage, also known as "The Leaf's Yellow Flash", was the brave ninja responsible for sealing the Kyubi inside Naruto. He is said to be the greatest ninja Konoha had ever produced and to have ever lived. He is remembered for his heroic acts through the monument that overshadows Konoha. Before he performed the seal, which would trap the Kyubi, he said that he did not want Naruto to be viewed as the container of a monster, but as a hero who saved the village from annihilation. This request, however, was not honored by the villagers, who still held much resentment for the Kyubi. He was a student of Jiraiya, and was the sensei of Kakashi Hatake, Obito Uchiha, and Rin.
The Fourth shares many similarities with Naruto, including blond hair, blue eyes, the same blood type (B), similar jutsus and element affinities, and overall similar personalities as noted by Jiraiya (though he also notes that the Fourth was much smarter, relatively speaking). Naruto is also noted to be the Fourth's legacy by Itachi Uchiha. Furthermore, when he was trying to force Naruto to release his Kyubi chakra, Jiraiya pushed Naruto off of a cliff, saying, "Forgive me, Fourth" as he did so.
The Fourth created several unique techniques, including Hiraishin no Jutsu, which essentially allows him to teleport to any location with the use of a special seal. He also created the Rasengan, a technique that does not rely on hand seals, but instead uses a concentration of spinning chakra in the palm that causes immense damage to its victim on contact.