Nest Keeper is a written note found in Dishonored 2.
Alexandria, I've always believed the idea of bloodfly "Nest Keepers" to be legend, or drunken miner talk, even though members of the Grand Guard have reported putting down "belligerent occupants" when burning out bloodfly infestations.
As such, this silvergraph represents a rare sighting. We can only conclude that the final stages of Bloodfly Fever occasionally result in an extraordinary kind of brain damage, wherein the host fixates on protecting the nests. The bloodflies in turn, welcome the afflicted person, and thus a symbiotic relationship results.
It can be found on a desk in Hypatia's apartment, next to the silvergraph it talks about, during the mission Edge of the World.