Karl Schäfer was a supporting character in Among Thieves. He was a former Nazi expeditionair who was sent to find the entrance to Shambhala, and secretly the Cintamani Stone, by the Ahnenerbe. Throughout the later events in Among Thieves, Schäfer serves as an advisory role to show Nate the true danger of Lazarević acquiring the Stone.
Schäfer appears as a playable skin in the multiplayer of Among Thieves. He is available at level 50 and costs $1,000,000. Schäfer re-appears as a playable skin in the multiplayer of Drake's Deception. He is only available through downloading the Classick Skin Pack #2.
Prior to Among Thieves 
Schäfer was hired by the Nazis to lead an Ahnenerbe expedition into Tibet in 1939, in order to find the lost city of Shambhala. In reality the Nazis wanted to find the fabled Cintamani Stone, something that Schäfer learned later. The expedition managed to discover the Phurba temple within the Nepalese city, as well as the ice temple deep within The Himalayas. Within the ice temple's inner sanctum Schäfer realized the truth regarding the expedition, and the dangers that Nazi Germany would pose to the world with the power of the Stone. Determined to stop ill-usage of the Stone's powers, Schäfer shot and killed the remaining expeditionary members, although he was heavily wounded in the ensuing firefight.
Near death, he managed to escape the temple and was found by people belonging to the local Tibetan village where he would spend the next seventy years of his life.
Among Thieves 
In 2009, Schäfer was made aware of an unconscious and wounded man being brought into the village, who was in possession of the Phurba. He realized that the search for Shambhala had started once more, which was further established when Elena came to the village looking for Nate, who told Schäfer about Lazarević. Schäfer wanted to meet Drake, and to warn him about the dangers of Lazarević acquiring the Stone, so he waited until Tenzin helped Nate recover enough from his gunshot wound.
The two finally met at the end of "Where Am I?", wherein Schäfer impressed on Nate the need to realize the truth about the Stone for himself, and had him set out with Tenzin as his guide to find the remains of his expedition. He also warned him that the Phurba was the last piece that Lazarević needed to open the passage to the lost city. Schäfer stayed within the village alongside Elena and Tenzin's daughter Pema, while still in possession of the Phurba.
During the soldiers' attack on the village in "Siege" and "Cat and Mouse" Schäfer and Pema were separated from Elena, and subsequently captured. As the soldiers are about to leave the village Pema is left behind, but Lazarević himself takes Schäfer along with him. Nate and Elena then witness him taking Schäfer through the monastery in "The Monastery". Nate and Elena finally manage to find Schäfer in "Reunion", although he had been tortured by Lazarević for information. Mortally wounded, with his dying breaths he encourages them to stop Lazarević and destroy the Stone. Afterwards, Schäfer succumbs to his wounds, perishing in front of Nate and Elena.
A memorial to Schäfer was set up in Tenzin's village, and Tenzin honored his memory with a Tibetan prayer.
Schäfer was multilingual, able to speak English, German, and Tibetan.
Behind the scenes
The fictional character of Karl Schäfer is based on Ernst Schäfer, who led a real life expedition to Tibet in 1938.
The photos Schäfer had was again from Ernst Schäfer.