Boris is Balalaika's second-in-command. He can be easily identified by the large scar running from the right side of his forehead to his left cheek. He is a calm, broad-built man with a deep Russian accent. Like Balalaika, he also saw service in Afghanistan. She still refers to him as "Sergeant" rather than by his name, suggesting that he was her former platoon sergeant. He is very loyal and protective of Balalaika and often worries for her safety when she decides to fight in the field. In an omake, he used to be a weak girlish looking boy before joining the army (this is unconfirmed however, as the omakes are non-canon.)

(Source: Wikipedia)