Karl Ruprecht Kroenen is a fictional character in the Hellboy comic book series, created by Mike Mignola. In the comics, Kroenen was a relatively unremarkable Nazi SS scientist, whose most distinguishing characteristic was that he always wore a gas mask and protective bodysuit, which Mignola attributes to a disfiguring accident of some kind. Director Guillermo del Toro created a significantly expanded biography for the character, to appear as a villain in the 2004 film adaptation.