A Fire Demon is a type of fantastic creature from the land of Ingary. They are extremely powerful creatures in magic, but are extremely weak physically. They originate as Star Children, or Falling Stars, who fall to earth from the heavens. Ordinarily, they are so weak that they reach the ground and have only a few moments before they wither and die. However, a human may catch one with the help of seven-league boots, and by entering a contract with the demon, may exchange their heart for some of their power. The contract grants the Fire Demon an extended life, as long as that of their partner's. However, it also binds the two together, and whatever one feels, the other feels as well.
Demons also seem to bring out the worst traits of their partner. Howl becomes much more self-centered and vain after his contract with Calcifer, and quits his schooling going into hiding. It is unknown if the Witch of the Waste changed after receiving her demon, or if she was always as cruel. However, in the movie, Madame Suliman mentions that she "fell prey to a demon of greed."