Calcifer

Calcifer is a fire demon from Diana Wynne Jones' novel Howl's Moving Castle. He is described as "a thin blue face, very long and thin, with a thin blue nose"... and curly green flames as hair. He has purple fire for his mouth, two green flames for eyebrows, and orange flame for his eyes. In the movie, he is a teardrop shaped orange flame, but in the scene where Howl moves the castle, Calcifer flares up and looks like his description in the book.

Spoilers
Calcifer is a fallen star who was caught by Howl. Calcifer didn't want to die, so he struck a proposed with the wizard. Calcifer got Howl's heart and a prolonged life, and Howl received full access to all of Calcifer's considerable powers. As it turns out, neither of them liked it after a while, but it couldn't be broken by people who were part of the contract. Calcifer strikes a bargain with Sophie later, in which Calcifer lifted the Witch of the Wastes curse as soon as Sophie broke Howl and Calcifer's contract. Sophie eventually discovers what the bargain is, and uses her newfound magic to give Calcifer another 1000 years of life, and to return Howl's heart.

Calcifers name is based on Lucifer.