Calcifer is a fire demon with a laundry list of talents. He can do just about anything. Let's be honest folks, Howl wouldn't be a tenth of the wizard that he is, without Calcifer. But does Calcifer get a "thank you," or a "Hey, why don't you take the rest of the evening off?" Nope. All of Calcifer's hard work is taken for granted by the entire cast of Howl no Ugoku Shiro. Calcifer is the Rodney Dangerfield of demons; he gets no respect.
Calcifer's work ethic is nothing short of amazing. He's a renaissance man that runs a castle 24/7 under horrible circumstances. Until the arrival of Ms. Sophie Hatter, Calcifer lived in a terrible work environment. Howl single-handedly made the entire Castle a pigsty. Your master turning into a giant green blob whenever he gets moody, can do that to a place.
When you first view Howl no Ugoku Shiro, you think that Calcifer is just a stereotypical side character there to offer comic relief. But sometime during your second viewing you'll have an epiphany. Calcifer is the unsung hero. Remember when Calcifer ate some hair and turned a bunch of wreckage into a mech? Or how Calcifer is constantly opening portals at the front door? You'd sound like Billy Crystal too, if you had to carry the entire weight of a castle on your shoulders.
When you view the film Howl no Ugoku Shiro from the perspective of Calcifer, everything falls into place. All of Calcifer's quips and insults are really cries for help. All he wants is a little recognition and some love. Instead all his work gets taken for granted. One of the reasons Calcifer comes to respect Sophie is because she at least tries to take care of him. She gets rid of his old ashes, teaches Howl some humility, and offers words of encouragement when things start to get tough. It's amazing what a little positive reinforcement can do to a fire demon's self-esteem.
Anyone who constantly loses employee of the month to the office pretty boy can sympathize with Calcifer. Your filing reports, working unpaid overtime, showing up fifteen minutes early, and yet no one likes you because they say your "snarky". That's Calcifer's appeal, and part of the reason why he is so popular among fans of Howl no Ugoku Shiro. Calcifer is more than a big mouth, he's a do-er. Sophie said it best, "They say that the best blaze burns brightest, when circumstances are at their worst."