Who is Boris Babrov from Russka: The Novel of Russia and what is their importance?

Boris Babrov lived in Russka during the 1500s. He is dedicated to Tsar Ivan, and he serves him throughout his life. Over time, he becomes a member of the feared Oprichnina. Boris will do anything to stay in the tsar's good graces, inclusing arranging the deaths of his father-in-law and son.