Drushka is a name mentioned in 2 Notes, The Outsider and Letter. The name sounds like a Rusalki name.
The Outsider is a green note, with a Rusalka author, but Letter is a purple note with an Athetos author*. Was Drushka really a double-agent, working for both sides?
I'm thinking that Drushka may be the missing author I have been looking for. Each Rusalki has unique speech patterns and there are a couple Notes that don't seem to fit any of them—English , Athetos. Athetos in particular is interesting Note as it states "he thinks I am dead". Certainly Athetos knows Elsenova, Ophelia, and Veruska are still alive, since he fought with them and is waiting for them to die, right?
Secondary question: How can "Letter" have an Athetos author? Which of his clones would not remember "that a man named Athetos came here once before"?