Brian was the lead actor in Marble Hornets, and a friend of both Alex Kralie and Tim. His character in the film was a college student, who comes back to his hometown to mend his relationship with Sarah and Tim, his old high school friends.
Apparences in the series
Warning: Spoilers from the series ahead.
Since the beggining of the series, Brian is missing. We only see footage of him back from 2006, when he was making Marble Hornets.
In Jay's interview with Tim in Entry 15, we learned that Brian lived near Alex and was the one who introduced Alex and Tim to each other. That interview also revealed that, like Alex, Brian has not been seen in some time. Tim also says that he was close friends with Brian and that he didn't know Alex before the shoot.
After seeing Advocate, Jay decided he needed to find Brian, and received an anonymous tip that he could find him at the house in Entry 16. Brian was never seen there, though later entries heavily supported the identification of the house as being Brian's.
Alex said in Entry 20, "So we're here on the set of Marble Hornets at Brian's house. Brian is somewhere; I don't know where…" He is also mentioned to be missing in Entries 17 and 22.
It is possible that Brian went missing (or "gone") some point between Entry 7 and 17. ~ Makdee
There has been some speculation that Brian may have been working with Tim to create the totheark youtube account. That even Tim could have killed Brian. But this is a rather unpopular theory as the evidence is very circumstantial, only that Brian appears in the Advocate Video and the bullet casing found in his home.
Could the blood seen in The House be Brian's?