Rosemary is a supporting character until the middle of the series. She grew up in an orphanage just like Nadja. Just like all the kids in there, when they reach the age of 13, they must work. So, when she reaches that age she goes to work, which leads her to an aristocrat house, the house of the Gonzales family where she works as a maid.
After some events, Rosemary gets obsessed with aristocracy and the life of those people, which makes her get to levels of madness, manipulation and sabotage that are scary for a 13 year old girl. Those events cause a lot of damages to her friendship with Nadja.