Marvin
Note: This is part of the version 2.0 update which you may or may not have yet. This wiki often contains information on updates before they arrive.
Initial Encounter
After buying the Diamondback and selling enough scrap metal to Johnny, Marvin can be found at the junkyard having an argument with Johnny Junks. Marvin will give the player his scrap metal and mention how to start the industrial shredder/forge which can create more valuable metal plates.
Second Encounter
After speaking with Marvin at the junkyard, he can be found again in front of a blue warehouse across the street from the Sheriff's office. Marvin will give you a vehicle recovery mission to replace Johnny's classic car he shredded.
Big Industry
After opening the Bar and delivering the abandoned vehicle to Johnny, Marvin mentions the bar you opened, offers to sell you the warehouse directly behind it, suggesting you convert it into a Brewery to supplement your bar. Marvin offers manufacture the brewery components, provided you supply him with enough metal plates.
Beer Recipe Perfection
Marvin can later be found at your bar after you have supplied the bar with beer you have manufactured. He will give you tips on what parameters need to be adjusted at the brewery.