Brian is the former boom operator of the agency that films the office.
Brian has been part of the crew since before the first episode. Pam says in Season 9 Episode 18 ("Promos") that he has observed them for 10 years. In a Season 5 episode, the boom briefly appear over Pam's head during a scene with her and Jim when Jim showed her their new house. This may have been Brian.
After a fight between Jim and Pam, Brian comforts Pam and tells her its a tough situation and that things will get better (Customer Loyalty).
Pam thanks Brian for helping her and wishes that they could continue to talk however Brian informs her the agency is cracking down on the camera crew talking to the office staff. When Meredith asks him when he will "boom" her, Brian seems happier to tell her the same thing before walking away. After Clark is moved to Jim's desk and he, Dwight, and Pam joke around, Brian is shown to be happy to see that Pam is happy (Junior Salesman).
When Pam is talking about how alone she feels after someone vandalizes her mural, Brian lightly taps her on the head with his boom causing Pam to smile and remember he is still there for her. After Pam and Dwight retaliate against the warehouse worker responsible for vandalizing her mural, the warehouse worker attempts to attack her however is knocked aside by Brian. He fights the warehouse worker off and saves Pam. Both are fired and Pam apologizes to Brian. Brian leaves but before doing so, tells Pam that if she ever needs anything, he is there for her. In this scene you really get the feeling Brian and Pam had something more. Almost like a hidden love, that maybe Pam doesn't reciprocate. however due to the crews invasion of privacy and perhaps her kids with Jim, these feelings never went anywhere.
After Brian is fired, Pam and Jim plan to meet for lunch with Brian and his wife, Alyssa, on Valentine's Day. At lunch it is revealed that Brian has separated from Alyssa, and Jim discovers that Brian consoled Pam when she was distraught over arguing with her husband. You can really see the worry in Jim's eyes when he learns this. After all he stole Pam away in the first place.
When the documentary promos reveal all the secrets of the office, Pam's workmates prompt her to seek out Brian in order to find out how extensive the damage. When she visits him at his apartment, (which the camera crew is filming their talk) Pam asks Brian's opinion of her marriage; She also asks about the footage, to which he responds that the crew filmed "pretty much everything." Incensed at this invasion of privacy, Pam storms out. Afterward, there is no mention of Brian again, except perhaps as an audience member during the Finale.