Boris serves as Balalaika's right hand in the organization. You'll recognize him instantly by the prominent scar cutting across his face from his right forehead to his left cheek. He's a stocky, composed guy with a thick Russian accent who's every bit as hardened as his boss—both saw combat together in Afghanistan.
The fact that Balalaika still calls him "Sergeant" instead of using his actual name hints pretty strongly that he was her platoon sergeant back in the day, and that dynamic clearly stuck around. He's fiercely loyal to her, often visibly anxious when she insists on putting herself in danger during operations. His protective instinct toward her is one of his defining traits.
There's an omake that supposedly shows he used to be a scrawny, delicate-looking kid before the military hardened him into the imposing figure he is now, though fans generally dismiss this since omakes aren't considered canon.
Content compiled by AnimeList.moe from publicly available sources.
