Ariana is the daughter of a loveless political marriage between an Antivan Crow diplomat and the then heir to the Ostwick throne, now teyrna of the city for the past 30+ years. Everything Ari learned of being an elegantly efficient rogue and occasional assassin when the situation calls for it, she learned from her Crow father. Everything she learned of leadership, diplomacy and manipulation in the Thedas game of thrones, she learned from her political animal of a mother.
In her youth, way back when, Ariana was very close with Sebastian Vael and Nathaniel Howe, whom she learned everything she knows of archery, drinking and whoring from.
(a headcanon that came from the irresistible proximity between Sebastian's background in Starkhaven, Nathaniel's in "the Marches" and the Trevelyans in Ostwick; don't dangle these delicious plot carrots in front of my face and expect me not to bite!) So they were besties in their spring days, but then things happened and the three got torn apart by circumstances; but whereas Sebastian and Nathaniel eventually found purpose in the Chantry and the Grey Wardens, Ari was left behind and pretty much drowned through her 20s, going nowhere and doing nothing of worth with herself. Eventually she turned into a sorry embarrassment to the family, being already well in her 30s without having married some creepy but rich noble or another and popped some kids to spread the Trevelyan power grip around, the only thing a fourth child in a noble family is good for.
(The only thing that keeps Ariana from being the real black sheep of the family is the third son - her excommunicated brother who we don't speak of because we never had a mage son in this family, no sir. Rafael Trevelyan, my human mage I will be romancing Dorian with, is at the peace summit at the beginning of the game with the representatives of his Circle, and dies in the big explosion. On his playthrough, Ariana will die in the explosion and he will survive instead. Both are deeply scarred by each other's deaths, because they still kept in touch in secret, and were each other's only family connection that actually ever felt like family. Very tragic. Much drama. Loves it.)
Anyway, a couple years ago Ariana's mother had enough of this tomfoolery of supporting a 30 year old alcoholic spinster trainwreck of a daughter's lifestyle of sleeping all day partying all night, and packed her off kicking and screaming to the Chantry, to make something useful of herself. She initially took on the meaningless title of Affirmed Sister (as Sten puts it in DAO - "So you were not a priest, did none of their duties, and took no vows, but you lived among them?") just to get it out of the way, but eventually, as the tale goes, Ari started to find some peace in the routine and some comfort in the faith, and started considering taking a more serious role in the Chantry like she heard Sebastian had, years ago, even though deep inside she knew this was not really what she'd ever wanted to do with her life. That's when she's sent to the summit with some of her older and more experienced relatives to represent the Ostwick Chantry, hoping to see her brother in person for the first time in decades and maybe sneak some time with him to talk about stuff, when everything goes awry and Ariana finally finds out what she was really meant for all along - kicking ass and saving the world, baby! 
Mage-Templar wise, Ariana is Templar-aligned and will definitely side with them. She doesn't hate mages or anything - just, being the average civilian raised in the Chantry's creed, she doesn't understand magic beyond the occasional scare that rogue apostates and blood mages cause, and the concerns of mages are not something that ever cross her mind. Even having a mage brother in the Circle who she keeps in touch with, this is all a completely alien conflict to her; all she sees, from the regular Joe point of view, is mages starting rebellions, killing the Templars that protect them, blowing up Chantries, harming innocent civilians, causing death and chaos and destruction etc, just as the Chantry always said they would if left to run free. As far as she understands it, these mages are only proving the Chantry and Templars right, and order needs to rule before they turn all of Thedas into the old Imperium again. (of course when I play her brother, the actual mage, he will see things very differently.
)
This should be a really interesting aspect of Ari's character for me to roleplay, though I'm technically super tired of this mage/templar stuff, because it turns out that some of my top favorite companions this time, and those who will definitely fit her personality the best for a friendship, are mages. I'm really really eager to see how working closely and becoming friends with actual, real life mages (not just as read and heard about on books and common gossip) will change her, and her outlook on many of her preconceived notions of how the world is supposed to work. This will be fun!