Following a lost cause, I'm afraid. My old friend Nephi fell in with a bad crowd. Drug runners, raiders, probably worse things it's better not to dwell on. When his gang headed west, I followed. (What brought you here)
Bert previously lived in Utah and is a member of the Followers of the Apocalypse, where he learned his skills as a doctor. Six months prior to the Courier's arrival, Bert left Utah and headed to New Vegas. He now uses this knowledge to help the residents in the Aerotech Office Park by being the slum's doctor and leader alongside Captain Parker.
Bert will comment on the death of Driver Nephi if the player speaks to him after killing Nephi, stating that he hopes Nephi's soul is at peace. No explanation is made as to how he knows Nephi, though cut dialogue exists which elaborates on this point (see behind the scenes).
If asked who he is, Bert will mention that within the Church he is known as "Elder Gunnarsson", making Bert one of the few residents of New Vegas to identify themselves as belonging to a specific religion; in this case, Mormonism.
"Poor lost souls, most of them. They thought to come to New Vegas and find their fortunes on its shining Strip. But the journey is often long, and the price to enter the city is steep. Those who could not pay ended up here, hoping to earn the caps they need. Some find honest labor, but more fritter away what little they've saved on crooked gambling and cheap liquor. Still, I minister to them as I can."
According to cut dialogue, Bert was at one point friends with Driver Nephi prior to his arrival in New Vegas, presumably when both lived in Utah. Nephi "fell in with a bad crowd," and when he and his gang moved to the West, Bert followed in the hopes of redeeming his friend. Bert states that he thought he "could turn [Nephi] back to the Church," indicating that Nephi at one point may have been a Mormon as well.