Karl Ruprecht Kroenen was a German Nazi scientist and SS Officer. After sustaining injuries during an experiment gone wrong, he was never seen without a protective gas mask and body suit. Kroenen had a fascination with experimenting on corpses, creating super-soldiers, and even attempting to cheat death – dark interests shared with fellow Nazi scientist and friend Herman von Klempt. He initially served Grigori Rasputin and his plans to bring about the end of the world. After failing to bring his master back from the dead, he allied with Varvara before being killed at her hands.

The character's backstory and design was greatly expanded upon by Guillermo del Toro for the 2004 film adaptation Hellboy, re-imagining Kroenen as an undying supernatural assassasin.