@Alistair Theirin was born in 9:10 Dragon, possibly in Redcliffe Village or nearby. Alistair was raised by Arl @Eamon Guerrin of Redcliffe brought up to believe that his mother was a serving maid who died in childbirth. As a child Alistair worked as a stableboy. The arl's wife, Arlessa @Isolde Guerrin suspected the reason her husband took an interest in the welfare of a servant's child was that Alistair was Eamon's son. Though her suspicions were unfounded, Eamon acquiesced when she demanded that Alistair be sent away to the Chantry. In 9:20 Dragon Alistair was sent to the monastery at Bournshire.
Despite the Arlessa's speculation on Alistair's parentage, he was in fact the illegitimate child of King Maric and the elven mage Fiona. In order to protect the honor of his sister, Queen Rowan, Arl Eamon had agreed to Teyrn @Loghain Mac Tir 's suggestion that Alistair should be sheltered in Redcliffe.
Alistair was trained to become a templar for many years by the Chantry, this being how he learned most of his martial skills. He was found to be ill-suited to a life of religious devotion, however. When Duncan, Commander of the Grey in Ferelden and a friend to King Maric and Fiona, found him in 9:29 Dragon, Alistair had not yet taken his vows and was desperately unhappy at the monastery. Alistair competed in a tourney held to honor the Grey Warden, being bested by acclaimed templars such as Ser Eryhn, a woman who wielded a sword and shield with unequaled grace, Ser Talrew, leader of many victories against the Chasind, and Ser Kalvin, one of the finest blades in Ferelden. Despite this, Duncan sensed that Alistair had a good and loyal heart, and so recruited Alistair into the Wardens. Alistair's recruitment was met with considerable resistance from the Grand Cleric, though she was forced to relent when Duncan invoked the Right of Conscription compelling the Chantry to hand Alistair over before he took the sacramental vows.
According to Alistair, Duncan saw he was unhappy and figured his templar abilities could be useful for fighting not only mages, but also darkspawn. Alistair always resented that people dictated the direction of his life and Alistair became grateful to Duncan when he saved him from the Chantry; since it was the first time Alistair found someone that cared about what he wanted. It's debatable whether Duncan pitied Alistair and wanted to rescue him or if he simply thought Alistair would be useful.
Although initially angry at the downsides of being a Grey Warden, Alistair grew fond of the Fereldan Grey Wardens and the Grey Wardens in turn grew fond of him. Alistair saw in the Grey Wardens kindred spirits—as all the Grey Wardens were once unwanted people welcomed nowhere until the Grey Warden cause brought them together. Alistair saw in the Grey Wardens a fellowship that encouraged his development as a Grey Warden and didn't care about his background. Coupled by Duncan's belief in his worthiness, Alistair grew to embrace the Grey Warden cause and flourished in their order.
As the most recent recruit of Ferelden's Grey Wardens, Alistair helps the Warden prepare for their Joining at the fortress of Ostagar. He accompanies the Warden-Recruits into the Korcari Wilds as part of their pre-Joining trials, encountering the apostate, @Morrigan, there along with her mother, @Flemeth, both of whom Alistair distrusts.
Alistair was given the task of lighting the beacon atop the Tower of Ishal. The tower is crawling with darkspawn by the time the he reach it, and though successfully fight his way to the top and light the beacon, @Loghain Mac Tir, teyrn of Gwaren and leader of King Cailan's armies, abandons his king and retreats. Alistair was overwhelmed by darkspawn and would have perished, but was saved by the intervention of @Flemeth.
Alistair is nursed back to health by @Flemeth and @Morrigan and is utterly devastated to hear of the defeat at Ostagar and Duncan's death, laying the seeds for a passionate hatred of Teyrn @Loghain Mac Tir. Following his recovery, Alistair set out to rebuild the army lost at Ostagar and defeat the Blight. Upon approaching Redcliffe, Alistair reveals the truth concerning his parentage. With the downfall of King Cailan at Ostagar, Alistair's bloodline means that he is the last of the Theirin bloodline, and a claimant to the throne.
Alistair longs to be reunited with Goldanna, the woman he believes to be his half-sister. With the aid of the Warden, Alistair eventually tracks Goldanna down, discovering that she is living as an impoverished washerwoman in the capital. Alistair's familial affection for his half-sister is not mutual, with Goldanna harboring a deep resentment toward what she perceives as Alistair's privileged upbringing and her belief that he had killed their mother.