History - Headcanon
Aisling was the princess of a group of the Tuath de Dannan (borne out by this picture, this one, and this one, all either endorsed or painted by the artist for the film, Tomm Moore). Her parents and her people were killed by Crom Cruach when she was rather young, for a fae. Perhaps the mental equivalent of, say, four, as she is probably about eight in the film. Hers was a happy childhood, with a wise father who indulged and disciplined her, and a mother that played with her and took care of her. She was taught to shapeshift, and her small spells, by either her parents or one of the more prominent members of her clan. Her family's death was no absent thing, to be returned to after a day in the forest, but happened in front of her, while she used her agility and stealth to stay out of the evil deity's way. Both her parents were warriors, and her people were no strangers to weapons, so it was a long and violent engagement. In the end, all their magic and skill could not save them, and they were struck down and probably eaten by the world worm.
The forest around Kells was not necessarily their original abode, but regardless, after she fled her home, she spent a great deal of time in other shapes - that of salmon, deer, and wolf, if not a few others (though she hasn't the full mastery of them). She traveled widely, but stayed on Ireland and in the seas there-about. She swam near the viking ships and saw the destruction they wreaked, and later in her travels saw them burn small towns and villages like the one Brenden's family lived in.
[Historically, the first viking raids were recorded around 790 and Cellach, Abbot of Iona, retired to Kells in 814. The Vikings continually raided the Abbey during the tenth century and it was repeatedly sacked and pillaged. Despite the constant raids, the monks managed to keep the Book of Kells intact until 1006 when it was stolen from the shrine. A reference in the Annals of Ulster is generally believed to refer the theft of the Book of Kells and it relates that the manuscript was returned after two months without its cover. So if Aisling saw the beginning of the viking raids on Ireland, and lived until, say, 20 years after the Book of Kells was ripped from its cover, that would make her at least 216, and that's only enough time to take her from age 4 to age 8, so double that, and she's probably 432 or so.]
She has never shown herself to other humans, or if she has, it's not likely that they are still alive to tell of it.
The forest around Kells was not necessarily their original abode, but regardless, after she fled her home, she spent a great deal of time in other shapes - that of salmon, deer, and wolf, if not a few others (though she hasn't the full mastery of them). She traveled widely, but stayed on Ireland and in the seas there-about. She swam near the viking ships and saw the destruction they wreaked, and later in her travels saw them burn small towns and villages like the one Brenden's family lived in.
[Historically, the first viking raids were recorded around 790 and Cellach, Abbot of Iona, retired to Kells in 814. The Vikings continually raided the Abbey during the tenth century and it was repeatedly sacked and pillaged. Despite the constant raids, the monks managed to keep the Book of Kells intact until 1006 when it was stolen from the shrine. A reference in the Annals of Ulster is generally believed to refer the theft of the Book of Kells and it relates that the manuscript was returned after two months without its cover. So if Aisling saw the beginning of the viking raids on Ireland, and lived until, say, 20 years after the Book of Kells was ripped from its cover, that would make her at least 216, and that's only enough time to take her from age 4 to age 8, so double that, and she's probably 432 or so.]
She has never shown herself to other humans, or if she has, it's not likely that they are still alive to tell of it.