Karl is partly a member of The Family and is located in the Meresti Station. He is only partly a member as although he provides the Family with equipment and aid, he is actually against Vance's teachings. He only plays along with The Family's goals because he wants his shop to be in the protection of Meresti Station.
His shop is only open from 2pm until 8pm. This is the smallest amount of hours for any shop in the Capital Wasteland.
He has one of the largest inventories in the game and you can buy a wide range of items from him. You are also able to buy Blood Packs from him for 5 caps and proceed to sell them to Vance for 15 caps. However, the Blood Packs only appear if you crafted the deal between Vance and the citizens of Arefu. Karl also has a large supply of Scrap Metal that you can steal and sell to various people in the game who require it.