Brian is a drug addict middle-aged man whom you encounter three times during the game.
After completing the mission “It’s Your Call”, an icon pops up on the map near Roman’s Taxi Company with a blue-human symbol.
On visiting it, you meet a guy named Brian Meech. Watching or skipping the cutscene will complete the mission.
After 3-4 in-game days, the 2nd encounter will pop up on the map.