Jiraiya

Backstory

Jiraiya is one of the three legendary sannin and at the time of the series is between the ages of 51-54.  Jiraiya started out as a pupil of the Third Hokage, along with teammates Orochimaru and Tsunade.  Some time after he and the other two left the Third's instruction they found themselves in a battle with Hanzo of the Rain village.  So impressed was Hanzo, with their abilities that he dubbed them the three sannin, or most powerful shinobi of the leaf village, and ended the current war.  After that Jiraiya encountered the Toad sages in his wandering, and found his natural affinity for the sage chakra.  After returning to the village, Jiraiya reached Jonin level and became the instructor of Minato Namikaze, who would later become the Fourth Hokage and Naruto's father.  Years later Orochimaru defected from the village and Jiraiya went in pursuit of him to bring him back (very much mirroring Naruto's work to bring Sasuke back).  However, unlike Naruto, Jiraiya decided that he could do nothing to save Orochimaru, and instead left the village in order to keep an eye on Orochimaru's plans.  This meant he was unable to take the role of Hokage as the third had hoped, and the role fell to Minato.