Aerin is the only daughter and oldest child of Ser Tomas Prinksett, Armsmaster of Clearwater. She's rather something of a tomboy, having been trained by her father in the use of Arms and Bow. At the prologue she'll be 13, and at the start of the game proper she will be 15, almost 16.
Her brother Tomlin left to Barrowtown to foster when she was 13 and he was 11. She and her father share quarters alone now. There doesn't appear to be a mother for Aerin at the start of the story.
She's had some informal training as a healer by the Septa Aloise, and has a small amount of Healing skill. But her independence and her love of archery will almost certainly steer her from that path and towards one more martial.
She's a dark, brooding teenager at the start of the prologues, mostly because of her resentment at the fact her brother has departed to Barrowtown leaving her stuck alone in Clearwater. With her father. And no friends. All alone.
Casting for Aerin is a very young Helene Bonham Carter.