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 ghoul.
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, and may even change from one type of ghoul to a stronger type with each re-incarnation, making them potentially extremely dangerous opponents for even the best armed and armored player (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 simply reloads a prior Autosave or Quicksave, they won't be in the same place.
A mailbox in the post-War Sanctuary Hills has the name Rosa printed on it. Her house however, was cut from the game.