Hellboy is a creature, summoned in the final months of World War II by Grigori Rasputin on Tarmagant Island, off the coast of Scotland, having been commissioned by the Nazis to change the tide of war ("Project Ragna Rok"). As a direct result of this ritual, he appears on Earth in a fireball in a ruined church in East Bromwich, England, on December 23, 1944. Proving not to be a devil, in the traditional sense, but a devil-like creature with red skin, horns, a tail, and a disproportionately large right hand made of red stone, he is dubbed "Hellboy" by Professor Trevor "Broom" Bruttenholm.
Taken by the United States armed forces to an Air Force base in New Mexico, Hellboy is raised by the United States Army and by the Bureau for Paranormal Research and Defense (BPRD), a private organization dedicated to combating occult threats. However, it is not until he eats his first human meal (a meal of pancakes) that he is forever bound to the mortal world.
As an adult, Hellboy becomes the primary agent for the BPRD, alongside other human and quasi-human agents that include Kate Corrigan, a professor of folklore at New York University; Abe Sapien, an amphibian humanoid (Ichthyo sapien); Liz Sherman, a young girl pyrokinetic; Roger, an unusually large homunculus; Johann Kraus, the spirit of a medium kept in a containment suit; and Captain Ben Daimio, a special operations man with occult experiences. The latter two have not met Hellboy due to Hellboy's resignation from the Bureau.
During a visit to Bromwich Church (the place of his "birth"), Hellboy learns he had been conceived 300 years ago by a witch, Sarah Hughes, and a demon Prince of Sheol. At this time, Hellboy had not existed as a baby in the "real" world; the prince's "favorite son" was considered to be "a power waiting to be born." Hellboy's "mother" also had children: a nun and a priest who would later haunt the church, dying in an attempt to stop the demon from claiming Hughes in her coffin. This origin indicates that Hellboy is, in fact, half-human.
A later, but possibly unreliable account gives the name of the demon as Azzael, and of the witch as Sarah Hughes. The spirit of Morgana le Fay claims that Hughes was her descendant; if this is true, it means that Hellboy is the last living heir to Arthur Pendragon and the rightful king of England. Hellboy has since drawn the sword Excalibur, claiming his position, albeit without much conviction.
Hellboy was granted "honorary human" status by the United Nations in 1952, and is known as the "world's greatest paranormal investigator". As such, he interacts regularly with humans, primarily law enforcement officials, the military, and various "scholars of the weird", most of whom are not presented as overtly reacting to his strange appearance.