Boris Vasily Konstantin Andrei Myshkin Dolokhov is Baron Wulfenbach's secretary and administrative second-in-command, who dutifully takes temporary command of the empire whenever Klaus is incapacitated, which means that he is running things during much of the Siege of Mechanicsburg. Along having four arms, he is brilliant, loyal and completely humorless.
Not much has been said about Boris in-comic, but according to various supplementary material, Dolokhov and his eidetic memory had been happily serving as a palace librarian until his Duke, a twisted Spark, decided to give him a pair of extra arms, enhanced speed, strength and balance... to become the "ultimate juggler". His unhappy tenure as a jester ended when the Duke and a neighboring Prince violated the newly-established Baron's Peace and were promptly quashed. Boris himself was picked out of the
victims survivors of the households by Baron Wulfenbach, and with Klaus's usual eye for "the right monster for the job", was set up in Administration, where he has succeeded brilliantly.
However, being part of the Baronial household is not without hazards. Boris's enhanced talents extend to swordwork as well, and at Baron Wulfenbach's side, he has faced far more unnatural Things than he cares to remember (not that he can forget anything anyway). Unnatural Things disturb Boris's sense of tidiness and order, but it's far better than being an enslaved clown.
Speaking of Unnatural Things, he and the Jägers have a love/hate relationship, as in: They love to pick on him, he hates (and being presumably one of a kind, is envious of their cheerful comradery.) At one point a General calls Boris . What this could mean is up for fan speculation, but it most likely is just the general greeting Boris, as Gospodin in Russian is a somewhat archaic form of respectful address, roughly equivalent to "Mister", and said General has vaguely Slavic name as well.
Following the events at Sturmhalten, the Baron's resulting stay at the Great Hospital at Mechanicsburg and the aforementioned Siege, Boris has been seen asking furtive and urgent of head-in-a-jar Rudolf Selnikov regarding the capabilities of Slaver wasps.
After the Klaus-induced Timeskip and Gil's assumption of control of the (remains of) the empire, Boris is very noticeable in his complete and unremarked absence from the Wulfenbach corridors of power, until finally (and literally) in Paris during Agatha's visit to that city. Or more precisely, her visit to the kingdom of the Argurons underneath Paris: he assists Larana Chroma's kidnapping of her own father, and the transport of his unconscious body to the Immortal Library. Boris then disappears again without any explanation of why he is there, or in how official of a capacity he is acting.
When Gil returns from his own trip to England to confront a Polar Ice Lord invasion, he Boris waiting for him in Paris, saying that it is long past time that Gil take charge of his family's empire.
The version of Boris that appears in the Girl Genius print novels is pretty much the same as in the comic, with as noted some more background details filled in. Agatha H. and the Siege of Mechanicsburg makes it explicit that he immediately sees holes in Klaus's very dramatic and public Gil has been a revenant for years, and that his questioning of Selnikov is part of his covert investigation of the situation.that
Possibly relevant outside information
According to The Secret Blueprints, Vol. I, Boris, whose middle names include "Vasily" and "Andrei", plays the balalaika. The Wikipedia entry highlights as epochal Vassily Andreyev's contribution to the development of the instrument.
"Mishkyn" is the last name of the protagonist of Fyodor Dostoevsky's novel The Idiot.
Fyodor Ivanovich Dolokhov is a character in Leo Tolstoy's War and Peace.
Although all his individual names are Russian, the naming scheme does not follow Russian tradition of one personal name, patronimic and one surname (double surnames exist, but they're hyphenated). Whether this means something (such as being a patchwork construct, like Baron, with individual names inherited from everyone used in a process) or just indicates a lack of research by Foglios is unclear.