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. However it is also stated he and Tim were also at a point 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. Arguably, 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 The Hooded Man. Note: Brian is dead, as he fell from a high floor in Benedict Hall in the later entries.
It can be inferred by the fact that Brian used Stereoscopic Colored, Artifacted and Glitched editing in his Totheark videos that he was aware that Tim would be unable to view his videos although they should have been able to see each others uploads. This is because Tim has Epilepsy and takes Seizure Medication likely Benzodiazepines, which can be used to treat Seizure disorders, Anxiety disorders and tics. However Brian had taken these medications throughout the series and entries. I cannot be confirmed if the medication Tim was prescribed was Benzos however due to the fact he suffer from Seizures and has Anxiety it is a great possibility.
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.