Auron’s past is something out of a medieval novel. When he was younger, around 24, he was part of the Warrior Monks of Bevelle, a prestigious group that trained its members from when they were small children in the art of the sword and holy scriptures of Yevon. Auron probably rose through the ranks quickly and was a very skilled swordsman. He was offered a promotion, but there was a catch. The current priest wished for Auron to marry his daughter, and offered a marriage proposal unknown to anyone else. Auron refused because he did not love her, and the enraged priest excommunicated Auron in return for his actions. From then, Auron met Braska, another man who had fallen from his position because he had married an Al-Bhed. By this time, Braska’s wife had died while going to visit her brother Cid in the Sandubia Desert due to Sin’s intervention. Braska decides to become a Summoner and recruits Auron as his guardian. Around this time, Jecht, Tidus’ father from Zanarkand, is transported to Spira, and arrested for disturbing the peace. Braska hears of him and is curious of the fact that he claims he is from Zanarkand. Braska and Auron go to visit Jecht and take him as a guardian despite Auron’s obvious dislike towards the man for being a drunkard. The three form a ragtag team for a pilgrimage, but still develop a strong friendship.
At the very end of the journey, when the three find out that the Final Aeon needed to destroy Sin must be made from a guardian and that the summoner must die with them, Auron’s resolve fails and he begs Braska to turn back. However Braska remains adamant about defeating Sin and says that they must go on. Auron would have been Braska’s Final Aeon, but Jecht had volunteered as he saw no way of returning home to his wife and son. Braska accepted this sacrifice and Auron was left hopeless. His two best, and only friends for that matter, were going to die, and there was nothing he could do about it. Braska made Auron promise to look after Yuna and bring her to Besaid Island, while Jecht asked Auron to find a way to Zanarkand and look after his son Tidus. Auron accepted both tasks and did not follow Braska and Jecht to the final room where they would defeat Sin.
Auron also feels deeply betrayed by Yevon, and after some hesitation, runs in, feeling his emotions overwhelm him, yelling at Yunalesca. He asks why Braska and Jecht had to die for no reason when Sin would come back. She replies that it is a neverending spiral of death and that a summoner must die to bring hope to Spira. Auron is not satisfied with this answer and attacks Yunalesca. He is crushed by her counterattack and loses his right eye in the process. He somehow makes his way back to Mt. Gazgazet and is found by Kimahri. Kimahri asks for his dying wish and Auron tells him of Yuna. Auron is later found by Rin, the Al-Bhed merchant who we see so much of in FFX. Rin tries to save him, but loses the battle and Auron dies during the night, becoming an unsent.
From then on, Auron searches for a way to get to Zanarkand, successfully making it there, and taking care of Tidus until the day that Sin arrives there and Auron takes him to Spira.
Additional information: It is now known that Gippel or Barlai met Auron during his search for Sin to get to Zanarkand. They even shared a little conversation, although it was just a little bit on one’s character. Whether it was Gippel or Barlai, I am not sure, but both of them did not know of Auron’s reputation.
Maechen, the historian of Spira, also met Auron during the pilgrimage. Why, no one can be sure, but in a cutscene from FFX-2, Maechen starts to fade away (he is unsent) and a hazy scene of the three men sitting by a fire is seen.