Calcifer

Calcifer is a Fire Demon in a magical contract with Wizard Howl. He used to be 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.

Calcifer is Howl's resident fire demon. As the result of a mysterious bargain with Howl some years ago he agrees to heat and power the castle. Although he is bound to the hearth, he has a great amount of magic. He promises to use his magic to break the curse on Sophie, providing she breaks the contract between him and Howl. Howl describes Calcifer as "his weakest point", because Calcifer wouldn't give away another demon if it entered the castle, even if it had hostile intentions.  However, Howl's statement is true in more ways than one.

Howl's Moving Castle (Book)
Calcifer first appears on Sophie's first night in the castle, although his magic can be seen earlier, as he is the one shooting off fireworks on May Day. He allows Sophie to enter the castle because he can sense her ability to talk life into things, which makes her a perfect person to lift the contract between himself and Howl. Sophie agrees, on the terms that Calcifer break her spell in return, and that Calcifer drops her hints about the contract, since he isn't allowed to talk about it directly. He does tell her that he cannot move out of the hearth, as the contract binds him there, and that he does most of the strong magic like moving the castle. He also says that his contract with Howl isn't doing either of them any good.

Calcifer's Hints
Because Howl is reputed to eat the heart of pretty girls, he says to Sophie: "Don't you seriously want your heart eaten?" Calcifer comments on this a number of times. "Howl is pretty heartless, you know." Howl has gone out to court Lettie. Calcifer replies to Sophie "Heartless Howl is finding this lady rather tough." Sophie comments "It's heartless and pointless" in regard to Howl's courtship habits. Calcifer replies: "He's made that way," Sophie has a pain in her heart, and says to Calcifer that he won't understand because he hasn't got one, he replies: "Yes I do!", as he in fact has Howl's heart. Calcifer also says to Sophie, "I’m scared too, I will suffer, if Howl does." After Sophie asks Calcifer if he was a fallen star: "When Howl offered to keep me alive the way humans stay alive, I suggested a contract on the spot."

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 and breaking the connection it had with Market Chipping in an effort to find and save Howl. By consuming Sophie's hair, he is able to have enough power to move the castle across The Wastes to find Howl.

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, who have been summoned to High Norland to investigate the Royal Family's disappearing money (Howl disguised as Twinkle, since the King cannot call upon the Royal Wizard of another country). Later in the book, he destroys both the lubbock and the eggs, as only a fire demon can destroy them.

Howl's Moving Castle (Novel)
In Howls Moving Castle and House of Many Ways, Calcifer has a thin blue face, curly green hair, green eyebrows, orange eyes and purple glints for pupils. In Moving Castle, he is restricted to the hearth by his contract with Howl, but has two very thin arms that can reach out a little ways. He also has Howl's heart, hidden under the logs. After he is freed from the contract, Howl takes his heart back and Calcifer is free to leave the castle, though he stays, because he "doesn't mind, as long as [he] can come and go." Without Howl's heart and contract weighing him down, Calcifer moves around by floating.

Calcifer is a fire demon, and as such is not always quite trustworthy, neglecting to tell Howl that Sophie and the dog-man Percivel were under spells, and refusing to warn Howl of another fire demon, even when it could have killed both him and Howl. However, he does keep his word when he promises to lift the spell of Sophie if she breaks his contract with Howl, doing it as soon as she nips Howl's heart off Calcifer's body. He also tends to whine a lot, and can be a little bit nasty. He is loyal though, choosing to stay with Howl even though the contract has been broken, and fighting the Lubbock because otherwise "[he] wouldn't deserve the life Sophie gave [him]."

Trivia

 * Calcifer actually takes on a form similar to his book form in the movie when he and Howl merge the Castle with the Hat Shop.