Brian is a friend of Alex Kraile, and also plays the main character of Marble Hornets, Brian.
Being a close friend of Alex Kraile, Brian played the main character of Marble Hornets, Brian as he attempts to adapt to his changed home town that he missed while in college. Its clear that him and Alex are good friends.
According to Totheark, Brian is an advocate, someone who stands on another person's behalf. In this case it is Alex.
Playing the main character, Alex wouldn't record any scenes without Brian present. Agrueably, it put Brian in huge danger for The Operator stalks Alex near every shoot of Marble Hornets. Brian has even shown signs of the sickness and has been in the presence of the tall being. In Entry 51, supposedly Brian's last moments the Operator attacked him and Alex, ending with Alex dragging his limp body somewhere in the building. However, its still clear that he is alive because he has been confirmed to be Hoodie. Note: Brian is dead, as he fell from a high floor in Benedict Hall in the later entries.
It was revealed in Entry 84 by Tim that Brian was wearing the same hoodie as the hooded man wears. In entry 86, Brian is shown slung against the wall, wearing the same beige yellow hoodie.