Alistair
Background
Alistair is the bastard son of King Maric (you come to find out this during
the game) and a maid, who he believes died when she gave birth to him. He was
raised (till he was 10 if I understood well) by Arl Eamon Guerrin of Redcliffe,
but when this one got married to Arlessa Isolde, she made insissted for him
to be sent to the Chantry, because she suspected that he was Eamon’s bastard
child. Eamon took him in because Loghain asked him, so they could protect Queen
Rowan’s honor (she was the wife of King Maric). Even if he is illegitimate,
after Cailan’s death, makes him possible heir to the throne of Fereldon.
Alistair was in the Chantry training to become a templar for ten years, because of that he already has a set of skills when he comes in to the play. Unfortunately, or fortunately for him, he was found quite unsuited for the life of prayers and similar. When Duncan arrives at the Chantry, Alistair has not yet taken the vows, and it’s said that he was also really depressed because of the environment he was in. Duncan was searching for new recruits for the cause (Grey Wardens), and even if he had more suitable candidates, when it came to skill, he still choose Alistair because he saw good in him, and also the fact that has a loyal heart, admiring him, because of that. So he used the Right of Conscription which states that “Should they need to, the Grey Wardens possess the Right of Conscription; they may demand that any individual from king to criminal be drafted into their ranks.« as to force the Chantry to give him Alistair, and it was done before he could took the vows, and even if the Grand Cleric didn't wish to give him up (I presume he knew he was the kings's bastard son, and wanted him under his watchful eye, and maybe use him later).
The truth about his birth

Now
that Inquisition is out, I stumbled upon the true lineage. As we all know he
IS the bastard son of the king, BUT he is not son of the maid and Goldana is
not his half-sister.
Stumbling on this information made me go blank for a moment, especially due to some things that happen in the Inquisition game.
First thing about Alistair is that Alistair is not full human, but just half-human, his mother is an elf, and she is a mage. She is Fiona the one that leads the rebel mages in the Inquisition. She was a Gray Warden when she had him, and she is an ex gray warden, because somehow she got cured, and she doesn’t know how either.
While Maric and Fiona had a love story, this couldn’t continue, still she got pregnant, and came to Maric with their son, asking him, to hide the true of his origins to the child, and not let him know about his elven heritage. She also asked Duncan to protect him. In DA:I if you get the mages on your side, and talk to Fiona, she will actually inquire about Alistair. Also, she faces him when he comes to Reddclife to ban the mages, and the fact is that he doesn’t know he is speaking to his mother, while she knows in front of her is her son… omg the drama. I’m looking at that with whole new set of eyes, and it makes my eyes water.
Now people, this information is mind-blowing, because one, if the warden from Origins is alive, he/she is searching for a cure, so they could end the calling, which the first one to do so was Fiona, mother of Alistair, even if she to this day doesn’t know that.
I’m still trying to cope with the fact that Alistair is HALF ELF.