Astrid Macmillan is an adventurous, curious, and thoughtful girl aged 11. She has shoulder-length black hair, brown eyes, and rather pale skin for someone who spends as much time playing outdoors as she does. Astrid loves to learn, but she prefers to touch and feel things in nature as opposed to reading about them from a book. However, this largely disappoints her parents, who prefer their children to be cultured and well-read.
Astrid lives with her parents and two elder siblings in a concealed townhome in London, England. Astrid's parents, Terence and Lyra Macmillan, come from a long line of pure blood families that include the Black, Prewett, and Greengrass families. Despite this, Astrid's lineage is unknown. Astrid and her sister, Nebula, were both adopted by their parents as young children. Astrid was adopted an infant and has no recollection of her life before the Macmillans, and she is rather content considering herself the youngest daughter of a pure-blood, upper-middle-class family.
While Terrence Macmillan is close to his extended family, Lyra Macmillan, nee Greengrass, has long been disowned by her prejudiced parents for marrying Terrence, considered a blood traitor, and welcoming children of unknown blood-status into their homes. Although their feud does not phase Astrid in the slightest, she does wish she had more magical cousins to play with. Although Astrid considers Nebula her best friend, the two sisters are polar opposites. Astrid is strong-willed and adventurous while Nebula is apprehensive and bookish. To this day, Astrid still does not understand how her soft-spoken, gentle sister managed to be sorted into Gryffindor.
Astrid herself will be starting Hogwarts soon, and she wonders if she will join Nebula in Gryffindor; join her brother in Hufflepuff, also her father's house; be sorted into her mother's house, Ravenclaw, or be sorted into Slytherin, the house of her maternal grandparents and their ancestors.