Nothing is known about the origins of Mercy MacGregor. But according to a portrait, she was the head Librarian from 1895 until 1910. She might have been the libraries first Librarian. During her tenure as the Librarian, she might have been a very strict and stern librarian who put up with no nonsense. She may have kicked out disruptive children or sent them to the basement to read inside the quiet reading room. Her basic belief was that Silence is Golden.
In the year 1910, she either died or retired from her job. Possibly when she died, she manifested into an insane and vengeful spirit haunting THE library to ensure that it would remain quiet and silent. Also She was granted a magical containment box, which she could use at her will to capture as many sounds or voices as she desired. Also she made the Quiet Reading Room a realm to capture people and their spirits.
When a person (usually a child) made noise near the Quiet Reading Room, she would open the door and approach them. Then she would use her magical containment box to steal their voice and trap them inside The Quiet Reading Room for as long as she desired.
Then for many years, she was dormant. This possibly happened because the door of The Quiet Reading Room needs to be visible for her powers to be in effect and overtime the basement got more bookcases and carts and the door was barricaded and no longer visible.
Then one day in 1994, a young boy named Timmy came down to the basement to retrieve his toy. He moved a cart out of the way and the door was now in clear view again which reactivated her powers. She opened the door, stole Timmy's voice and trapped him as a prisoner in the room. Then soon, she trapped another girl into the room named Natalie, after her little sister was making so much noise singing a song.
Then at nighttime, she would place the library into a time loop back to the year 1910. When this would happen, everything modern would disappear from the library and the library would appear to how it looked in 1910. Then one night, she could sense children wanting to break into the library so should unlocked an upstairs window for them.
The children were Laurie Napier and Jace Ellman. They decided to break into the library to retrieve her notebook to be able to finish their history assignment together. She soon encountered them, believing them to be making too much noise. She stole Laurie and Jace's voices and tried to trap him into the reading room with her other prisoners.
Then soon Laurie and Jace kept playing with a sound effects toy that made loud and irritating horn and siren sounds. This irritated her beyond belief as she struggled to capture so many sounds. Then eventually her magical containment box overloaded and broke down and all the spirits were released. When this happens she lost her powers, and she was destroyed. Then the library was peaceful and no more noisy children were ever at her mercy again.