History and headcanon!
Nothing is known about Rosalia's very early life, but presumably her parents died when she was young and she ended up in an orphanage in Maine. There Rosalia earned the nickname "Crybaby Rose" because she was small, sad, and sensitive. Her life turned around, though, when an older girl at the orphanage named Marria Torres accidentally set fire to a bathroom while Rosalia was inside. Maria ran off once the fire started, but once Rosalia started crying she came back at once and rescued her. Rosalia was very thankful for this, and adored Maria afterwards to the point where she referred to Maria as her big sis. Maria left the orphanage soon after that, but Rosalia never forgot what she did. This was probably a big thing for her, and may have marked her change from sadness to the optimism she later displayed in life.
Soon afterwards Rosalia was adopted by a virology researcher and college professor, Albert Sartre, who discovered a powerful virus contained within her blood and an equally powerful antibody that protected Rosalia from its effects. (Albert's reasons for adopting Rosalia are unknown. He could have discovered her blood's properties and then adopted her, but he could have just as easily adopted her and happened upon a girl with a crazy virus/cure in her blood: he'd already adopted another boy in the past and all indications point to Albert being a very loving father. I lean towards this second explanation.) She had a brother*, a boy Sartre had adopted years before, who soon began to study at the same college where their father taught. Rosalia...adored them both too.
Once Rosalia joined the family and Sartre knew of her blood’s potential, their father began to research the virus in her blood. If he could control its harmful effects and utilize it as a cure, he could potentially come up with something that could heal any disease, from cancer to the common cold. Within a few years, though, working with the disease drove him temporarily insane, as one of the virus's effects was madness. What happened next is mostly unspecified, but the entirety of Cumberland College died of the virus save for his son, in whom he induced amnesia with some kind of drug and whom he left to take the fall.
The madness was only temporary, however, and Albert soon came to his senses. He fled the country with Rosalia, who knew nothing of what had happened and likely thought they were moving to Mexico so Daddy could continue his research without being distracted. For years they lived in a house doubling as a laboratory surrounded by a field of flowers that Rosalia herself populated (Rosalia herself describes it as a "garden" in a letter written to Maria, but it's a liiiiittle more than that), while Albert continued his research on Rosalia. Despite the isolation the two of them were very happy. Albert and Rosalia cared for each other a lot, and knowing that her father was making a cure to save everyone Rosalia was willing to endure through the numerous tests he put her through.
Six years later, though, Albert had been having a tough time with the research, as eight years of research had failed to neutralize the virus's threat. He suffered from the virus in full this time, and likely went mad from it a second time. This time he sought to end it all and save the world from the virus-by killing Rosalia. Rosalia died in the flower field outside their lab, but that wasn’t the end.
The flowers she died in absorbed her blood, and the monarch butterflies that fed on it passed the virus they'd picked up on throughout their migrations. The result? The beginnings of an epidemic spanning from Canada to Mexico. Rosalia lived on as a spirit of some sort and likely watched all this unfolding, almost entirely as an observer. (Supernatural workings? Don't ask me, this game has three different explanations as to how people get or use the Healing Touch, not counting the artificial one. Consistency on the supernatural does not exist.) Personal speculation says she kept an eye on the major players and some of those infected by the virus (which could partially explain why victims mysteriously knew of her name), and found out a little more about what she hadn’t known about her father and brother this way. After a few years Maria was able to see her, but even then Rosalia could communicate little.
She had no power to put a stop to it all, but she didn't need to. Her brother, who'd been released from prison temporarily to work at the same hospital Maria (a paramedic) worked at, regained his memory after coming into contact with the virus. He spread the news about who Rosalia and her father were and how they were related to everything, and from there they located her body in Mexico and eventually were able to procure an antiserum to counter the virus. It was solved; the virus finally had its cure, and the threat that Rosalia and Albert had ended up unleashing was gone.
* Rosalia's brother is never named in the game, being amnesiac for most of it--he goes by his prison number, "kid," "that surgeon," etc. Rosalia only refers to him as "my brother." However, supplemental Japanese materials supposedly definitely indicate that his real name is something like Erhard Muller.
Headcanon
- One of Rosalia's primary form of entertainment while living in Mexico was books, which her dad would bring back every once in a while. She never learned Spanish, though, so he had to go get books in English.
- Another way she spent her time was trying out craft-y things. She learned a little sewing and likes to come up with new ways to look nice--she wears ribbons in her hair because it was something she tried out on a whim and her dad told her they looked nice.
- Rosalia loves dressing up and getting to look nice! It's fun for her. Her father's doting only encouraged this.
- Rosalia was for all intents and purposes completely isolated from everyone except her father during the years she and Albert spent in Mexico. Her dad went out once in a while for groceries and supplies and whatnot, but she was to stay home all the time.