This is the fellow who is operating or at least watching the Monolith portal-gate in The Refuge of Storms when Agatha, Tweedle and Co. come unexpectedly tumbling through said gate following their two and a half year trip from Mechanicsburg. The fact that he is reading a book, eating a sandwich and has a pillow ready to deploy would indicate that this duty is (or rather was) not terribly strenuous. Following the gate's blowing up, Tweedle's attempt(s) at getting information from him are interrupted first by Agatha, and then the arrival of Leopold and his bodyguards. Even if Borislav has survived Tweedle's bloody reclaiming of the Refuge, the story's action has moved on, and his moment in the spotlight has probably passed.
The word gospodin is a Russian honorific like Mister, in English.