Who is Lena?

In a scene that begins around 1.13 minutes in, Artem is in the apartment of a woman he calls Lena. She has a son she calls "Slava", who is the character recruited to "hunt deer" at the end of the movie.

What is Artem's relationship with Lena? His wife? Her name is only mentioned once... They just don't seem to be husband and wife.