Calcifer

Calcifer is a Fire Demon in a magical contract with Wizard Howl. He was originally a falling star, whom Howl was able to catch before he fell to earth and extinguished. He is a very powerful creature himself, with a great deal of magical ability, though cannot move beyond the confines of the hearth in which Howl keeps him without the wizard's help.

He appears as a teardrop-shaped flame, blue in the books and orange-red in the movie, colored differently in different places in such a way as to make a face. He has tiny licks of flame that serve as arms and legs, though they are not very large and hardly useful.

Howl's Moving Castle (Book)
Calcifer makes a deal with Sophie to get rid of the contract that he and Howl have in order to break her curse. During the book, he gives Sophie various hints on how to break the contract. Later, he also tells Howl that the Witch of the Waste had discovered his sister's family in Wales. He also has a part at the end of the book, where the magical contract is broken. But he returns to the castle at the end.

Howl's Moving Castle (Movie)
Calcifer's role in the film is somewhat similar to his role in the book, moving the castle. Unlike in the book, however, he is forced out of the hearth by Sophie, causing the castle to collapse.

Castle in the Air
Calcifer has a short appearance at the end of the book. It is also revealed that he was the flying carpet.

House of Many Ways
Calcifer appears along with Sophie and Howl. Later in the book, he destroys both the lubbock and the eggs as only a fire demon can destroy them.