Scientist - takes care of every technology in Hideout. You have to upgrade him in order to open new Story missions and upgrade your co-workers.
Gunsmith - Upgrades your weapons and lets you build more weapons from blueprints
Engineer - sells and upgrades chicken gadgets, explosives, and automated weapon systems
Smuggler - Sells Health, Damage, Money and Speed Boosters for Gold. You can upgrade him in order to get a discount on all Smugglers goods.
Medic - mixes up painkillers, assembles medkits or prepares auto-heal for you (and for a price).