Karl is 37 and was married to a woman named Skyler. Both of them lived together in Meresti before she passed. They were once owners of a shop in Megaton, and Vance met them while gathering supplies. He had felt a strong kinship with them and invited them to join him. Karl is very gruff and has a short fuse; he prefers not to speak to people. He is always highly suspicious of people and prefers to think the worst of them. Karl is distinguished by his unusual haircut: long blond hair, which is not common for a man of his bulky stature.
Karl doesn't believe in Vance's vampiric teachings, but plays the part in order to keep his store in the Meresti Metro Station.
His store is open from 3 pm to 8 pm (unlike other stores, open approx. 8 am to 6 pm). He will sell to you at other times if you find him. He sometimes sells blood packs at the regular "junk item" price (base value 5), which can then be sold nearby to Vance for 15 caps (not subject to Barter). This is only possible if you have brokered a deal with Vance and Evan King of Arefu.
Karl also has one of the biggest merchant inventories in Capital Wasteland. His inventory is located in a freezer near his desk.
He stocks around twenty pieces of scrap metal at the start of the game.
Unlike anyone else, Karl never runs out of stimpaks to sell.
With the Black Widow perk or a high enough strength level, Karl can be "convinced" into giving the password of the security computer terminal.
If you listen to Vance's conversations with him, you find out that Karl is the one responsible for the slaughter of Arefu's brahmin.