This is public information.
The Keepers are ghostly figures that wander around the Librarium at all times, going about their own mysterious business. Dressed in identical tattered robes, there is no way to tell one of them apart from another.
Most people assume that the Keepers are the ones responsible for “disappearing” the people who break the Librarium’s two rules regarding books, but it has never been proven. For the most part, people leave the Keepers alone, and the Keepers do the same.
The only time a Keeper is seen doing anything but shelving books is when a structure is placed so that it obstructs common access to the shelves. When this happens, Keepers are observed dismantling and removing the structure within a week.
This is privileged information.
If you try to attack a Keeper, it just collapses into a pile of rags and clothing which, after an indeterminate amount of time, disappears. According to the few brave souls who peered into their cowls, there is absolutely nothing under the robes: just darkness.
Till date, nobody has dared to try set fire to a Keeper’s robes.