A Forgotten Echo

A girl with a very unique name enters a city of monsters. War between humans an monsters has started after the humans killed Frisk. This girl, her name is Fallen Demon, and she has a cold when she enters the city and begins living in a pharmacy. Monsters dont use it, and with no lower windows, it makes a great home. She takes madication to help her get better, and ventures out at night to get things to entertain herself, like books and food that isnt just snacks. She soon begins to be known amongst the monsters as the demon thief, which she finds funny due to her name.
Gaster!Sans is sent after her, to find her and capture her alive, by Undyne. He goes to the abandoned pharmacy, and Fallen having heard him enter goes to hide. Sans stakes the place out, finding signs of someone living there. He checks everywhere, and is about to leave until he hears a cough. Of course he finds her.
But things soon take a turn for the worst and it becomes uncertain as to whether the girl known as Fallen Demon will survive.