Before the war, Ms. Rosa was presumably one of the neighbors of the Sole Survivor who didn't make it to safety. She can be found 210 years later roaming the Commonwealth and Boston, with her son and a few other unlucky pre-War citizens turned into ghouls.
In a terminal entry she is mentioned as a customer of a chems dealer who lived in Sanctuary Hills at one point, buying Buffout from them. She was also a widow and was restoring an old Corvega with her son.
The encounter with Ms. Rosa and the other ghouls can repeat, even if the Sole Survivor has killed them previously. These ghouls will level up with the player character, and may even change from one type of ghoul to a stronger type with each reincarnation, making them potentially extremely dangerous opponents for even the best armed and armored player characters (they will always spawn together). Fortunately, these monsters only re-spawn sporadically, meaning that even if the Sole Survivor falls to them in battle, defeats them, or if the player character simply reloads a prior autosave or quicksave, they won't be in the same place.
In the Red Rocket truck stop, she is mentioned in the terminal, and whoever entered the log mentions liking her.
In cut terminal entries, it is revealed that her husband died and that she and her son Louis were rebuilding a Corvega together, strengthening their relationship after the loss. Louis wanted a roll cage and a leather interior for the car.
A mailbox in the post-War Sanctuary Hills has the name Rosa printed on it. The house was cut from the game.
↑"Kind of crazy to think of how far Louis and I have come these past four months. And I'm not just talking about the car. I hate getting all poetic, but rebuilding this Corvega really has helped rebuild our relationship. He still misses his dad - God knows we both do - but he's learning that his madre ain't so bad. As for the Corvega: Most of the body work is done, generally speaking. Might not be pretty (yet!) but it's there. Interior needs new upholstery. Louis insist on leather - we'll see. - Louis wants a roll cage. I don't know how fast he wants to go, but I promise him we'll at least achieve the baseline 0-60 in .5 seconds. "