"Auron (アーロン, Āron) is a thirty-five-year-old warrior, attacking with various blades that fit into the Katana category. Shortly after the disappearance of Tidus' father, Auron became something of a mentor for Tidus, watching over him from afar. In the game's present day, he - along with Tidus - is Yuna's guardian, as he was to her father. It is eventually revealed during the game that Auron attacked Yunalesca in a fit of rage after she revealed that Braska and Jecht needlessly gave their lives against Sin, and that she struck him down with a mortal wound. Despite his injuries, Auron manages to literally crawl all the way to Bevelle before his strength abandons him. He meets Kimahri outside of the city, and requests that Kimahri fulfill a promise that Auron made to Braska in his stead (that Yuna be moved to Besaid and have a peaceful childhood), before he dies. Afterward, he became an unsent, and secretly held this status for nearly the entire duration of the game (this is why Auron would not enter the Farplane at Guadosalam in Final Fantasy X as he may not have been able to physically leave after entering). Soon afterwards, Auron takes advantage of his undead status and rides the reborn Sin to the Dream Zanarkand, where he watches over Tidus until the present day, where he and Tidus are transported to Spira at Jecht's request. At the end of the game, when Yu Yevon is defeated, he is finally laid to rest. Auron was also once a warrior monk for Bevelle, but he holds no connection to the teachings of Yevon during Final Fantasy X due to what he sees as the pointless sacrifices of Braska and Jecht, and the futility of the Final Summoning. A Jecht Sphere reveals that he rejected the love advances of a priest's daughter, which damaged his reputation. Auron has a brief appearance in Final Fantasy X-2, where his voice helps Yuna during her battle in the Farplane with Vegnagun.
Auron appears in Kingdom Hearts II as a one-world ally in the Olympus Coliseum. In Kingdom Hearts II he is brought back to life by Hades in order to kill Hercules, much like Cloud Strife's role in the first game. But unlike Cloud, Auron flatly refuses to help Hades, and joins Sora, Donald Duck and Goofy in battling Cerberus. He is later brainwashed by Hades into fighting Hercules in the Underdrome, but during the battle, Sora, Donald and Goofy restore Auron's free will, and he joins them and Hercules for the final battle against Hades, which ends with Hades falling into the River Styx.
A chibified version of Auron appears in Itadaki Street Special with Tidus and Yuna as playable Final Fantasy X characters.
He is ranked fifth on EGM’s Top Ten Badass Undead."
Source: Wikipedia, "Characters_of_Final_Fantasy_X_and_X-2", available under the CC-BY-SA License.