Karl Ruprect Kroenen

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 supervillain in the 2004 film adaptation.

Dr. Kroenen became one of the top scientists for Project Ragna Rok, and a close disciple of Grigori Rasputin, along with Ilsa Haupstein and Leopold Kurtz.

Kroenen, Haupstein, and Kurtz were frozen inside a secret Nazi base, until they were resuscitated by industrialist Roderick Zinco, acting on Rasputin's orders.

Kroenen assumed the body was intended for Rasputin before learning that Marsten tricked him into resurrecting the Black Flame, escaping the resulting chaos with von Klempt's head.

Born in 1897 Munich, Karl Ruprecht Kroenen was an opera prodigy with angelic features and blonde hair who toured the capitals of Europe until his career ended once his voice deepened with the onset of puberty.

He was awarded the Iron Cross for services to the Third Reich, including a tour of duty as commandant of Auschwitz concentration camp, where he served with distinction.

When Allied Forces stormed the island off the coast of Scotland where Project Ragna Rok took place, Kroenen killed several of the American soldiers attacking the base, but was distracted by a grenade thrown under the portal device by a young Professor Trevor Bruttenholm.

Kroenen tried to retrieve the grenade, but his left hand was blown off, and a length of concrete reinforcing rod (or rebar) impaled him through the chest, severing his spine.

Thanks to the inexplicable powers of science and black magic, Kroenen "repaired" himself with a prosthetic mechanical hand, a steel rod replacing the broken part of his spine and a clockwork heart, operated by a wind-up key implanted in his chest.

After long decades, the blood in his veins dried up completely, leaving only dust, and rendering him virtually invulnerable to gunshot wounds.

Hellboy seemingly avenged Bruttenholm's death by throwing Kroenen into an impalement trap – a spiked pit hidden beneath a trapdoor.

In the commentary of Golden Army, Guillermo del Toro also stated that, in the planned story for the third film, Kroenen was to have some history with the character of Johann Krauss.

German film historian Florian Evers devotes a whole chapter of his book on National Socialism in modern pop culture to the character of Karl Ruprecht Kroenen.