Born to the pure-blood Mercy family, an English family with Danish origins, Rosalia found herself to be quite the academic in most things scholarly, though consistently had trouble with magic; she often finds herself frustrated that her older siblings seemed to take to it quite easily, even before attending Hogwarts. She hopes to find the cause for her ineptitude upon enrollment and live up to the family name and role expected of her by her parents. 

Quite tall for an eleven-year-old girl, standing at around 163 centimetres with no sign of her growth halting any time soon. Due to her early growth spurt, her frame is very slight, and wiry, not yet filled out to her size, though her family members all tend to carry a little extra weight hereditarily. She has honey-blonde hair and fair, milky skin and, like the rest of her family, has silvery-blue eyes under heavy lashes. She wears a thick pair of white-framed glasses and is near blind without them. 

Rosie comes from a long line of Ravenclaw members, though her siblings often teased her, stating that she belonged in Hufflepuff for her consistent care of animals and general introverted, anxious personality. This got under her skin as she always felt herself to be a well studied academic, though there was a part of her which hoped they were correct and that she ends up in Hufflepuff if only to keep herself clear of her older brothers and sister already within Ravenclaw. To Hogwarts, Rosalia brought her dearest friend: a smoke coloured Nebelung cat called Brynhildr, or Hilde for short; with whom she is inseparable.

Sure enough, the Sorting Hat classed her into Hufflepuff, though did hesitate a moment, edging towards Ravenclaw before making its conclusion. With a secret smile, she collected her things, avoided the looks from her older siblings and began to unpack in the Hufflepuff common rooms, away from her siblings for the first time in her life; feelings of both anxiousness and relief washing over her.