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.
Calcifer is powerful, but can be just as cowardly as Howl, preferring to run from the Witch rather than fight her. However, he, also like Howl, will fight when the need arises. He is also fairly crabby and a little mean-spirited, which stems from being bound to the hearth in the moving castle for over five years. He has a natural fear of water, and also worries quite frequently about running out of logs, which are essentially his food. Sophie seems to be the only one capable of forcing him to do anything he doesn't want to do, a trait she also extends to Howl, and to a lesser extent, Michael as well.
He is also the first one to recognize Sophie's incredible ability to talk life into the world around her, which is the reason he allowed her into the castle in the first place and was so eager to make a bargain with her - if anyone but she were to break the contract he had with Howl, then Calcifer would die. Fortunately, she is able to talk life into him, and he survives losing Howl's heart, even deciding to continue living with them after he is freed.
Read more about this topic: Howl's Moving Castle, Characters