As a Young Man
"I wanted to change the world...but in the end, I changed nothing."
Throughout FFX we learn very little about Auron's past--mostly just spheres and the occasional jaded comment from the man himself. What we know for sure is only this: Auron trained for many years as a warrior monk until Lord Braska asked him to be a guardian. At this point Auron was in a bit of hot water, having just refused the high priest's daughter; thus it wasn't too difficult for him to drop everything and follow his friend to imminent doom.
Friendship, I would imagine, was pretty darn important to Auron, especially since he had so few friends. A fellow monk named Kinoc obviously held a lot of respect for him at one point, as we learn in the Auron sphere. But after taking a position originally intended for Auron, Kinoc essentially throws in his lot with the--er--slightly amoral Seymour. It's hard to believe Auron would have been too close to a man who so quickly gave up on protecting the lives of the people of Spira. Or maybe they were really close after all, making Kinoc's betrayal that much more bitter for Auron.
For a man who always thought pretty highly of himself, Auron was not so arrogant as to be unable to look up to anyone. He deeply admired and cared for Braska, and I often wonder why. Did they know each other before Auron became Braska's guardian? Or was the mere fact that Braska wanted the protection of an outcast warrior monk enough to endear him to Auron to such a degree? I mean, Auron essentially holds off death to fulfill his promise to Braska, and to ensure his dear friend's fate never befalls another summoner ever again. That's dedication.
I imagine Auron as a loner of a young man, slow to trust and slower to crack a smile. He did things by the book, studied and trained hard, filled his head with the naive idealism of a man who believes it's his fate to change the world. He gave himself, body, heart, and soul, to protecting Spira by following the ways of Yevon.
Then Yevon betrayed him.
Angering the Gods
"If the temples try to stop us, then we will defy Yevon."
When did Auron's belief in Yevon start to fail? What must it have been like for him to watch Braska and Jecht, his two closest friends, sacrifice their lives for a cause Auron knew was somehow flawed?
The Jecht spheres show Auron as an inpenetrable fortress of strength. He rarely smiles. He's constantly berating Jecht. He has a clear, defined purpose: to protect Braska at all costs.
But then we see him begin to falter. As he tells Wakka and Tidus many years later, he began to seriously doubt upon reaching the Zanarkand ruins. He had known all along that this journey would be Braska's last. He had accepted that....Or had he?
What if there were another way of doing things? A way in which no one had to die? Once Auron felt this thorn in his side, there was no way to remove it.
When Auron faced Yunalesca again and demanded to know what his friends had died for, he bridged the gap between speculation and blasphemy. It wasn't Yunalesca he wanted answers from--he wanted answers from Yevon, that great deity who had allowed so much pain in Auron's life. And maybe, somewhere deep down where he'd never admit it, Auron wanted answers from himself.
Auron had devoted himself for years to the study and training of Yevon's teachings. He had believed fiercely, perhaps blindly. For most of his life, that had been enough. But now, in the wake of the death of everything he knew, Auron realized he had only one person to count on: himself. Not a god, not a monastic order, not even a dear friend. All of these made mistakes. From now on, he would not.
Self-reliance and determination persevered beyond death. Auron's sheer pig-headedness, his inability to leave things unattended to, prevented him from going quietly. For ten years he held off death, becoming older, grayer, and maybe a little wiser. He kept his promises. He bided his time. But he let no one in.
And it was better that way--or so he thought.
Life After Death
"We will protect Yuna from anyone. Even a maester."
Braska's daughter chose to follow in her father's footsteps and become a summoner. Jecht's son chose to follow in this father's footsteps and become a star blitzball player. For Auron, everything worked out nicely: now he could keep an eye on both of them, as he'd promised their fathers.
Joining Yuna's group of guardians, Auron acted as a father and mentor figure for the group, reluctantly giving out advice and guidance while revealing as little as possible about himself. If he could have, I think Auron would have stayed in this aloof position, caring but not really a part of the group.
But somewhere along the line, things changed. Maybe it was when Auron realized that where Braska had failed, Yuna would prevail. Seeing his young charges stand up to Yunalesca; knowing that they had the courage to face whatever needed facing to assure that Spira would survive in a better way; it must have been gratifying for Auron to finally sees his hopes within the realm of realization. Could it be that Auron started to look at these kids as his fellow guardians, and maybe even his friends? Auron took it upon himself to tell Tidus that Jecht loved his son, even if he never showed it. Auron assured Yuna that Braska would be proud of her. He admitted to seeing some of himself in Wakka. He caved in to Rikku and let her regain her courage to face the Thunder Plains. These aren't the actions of an aloof, legendary guardian. Whether or not he'd admit it (and my theory is that hell would have to freeze over first), Auron cared very deeply about the others, and became not only their advisor and hero, but their friend.
Rebirth
"Thanks for meddling."
While Auron's role in the Final Fantasy world may be over (his woefully under-used cameos in FFX-2 don't really tell us anything new about the character), he returned to badassery in Kingdom Hearts II, where he was caught up in Hades' scheming. Sora and the gang met Auron in the Underworld as he refused to be a part of Hades' plans. Auron wondered if perhaps Sora needs a guardian (the similarities between Sora and the wanky Tidus are not to be underestimated)--but, as fate would have it, Sora ended up being the one to help Auron out. When Hades kidnapped Auron's free will and forced him to fight Hercules, it was Sora and his buddies who returned Auron to his senses. Through this process, Sora came to realize how difficult Auron's life must have been, and how strong the difficulties had made him. Freed from Hades' clutches, Auron told Sora and the others that, with no one left to protect, he was ready to write his own story. Unfortunately, this didn't seem to amount to much other than dissolving into pyreflies and thereby (presumably) dying once and for all over the ending credits of the game.