Calcifer, not a real demon.
Calcifer's name definitely came from Lucifer. Miyazaki had Jesus imagery in NausicaƤ of the Valley of the Wind, but I don't know how much he knows about Christianity and what his religion is or if he is apart of one. The novel was written by a woman from a Christian family, so I wonder what her idea of the character was.
In the movie, I guess he's either a fallen star who has Howl's heart or he is Howl's heart (I wasn't sure). Ok, so I am Christian, but even I didn't know all this stuff, so I went to Wikipedia (and let's hope it's accurate and I've head this stuff other places, so I think it's right) and I guess Lucifer was a bad king and not the devil and there's stuff from other cultures that may have influenced how we see him and the devil.
I see Calcifer as being like Howl with his vanity, in the way that he doesn't have to be bad (a demon), he can be a light of hope (like when John the Baptist said Jesus would baptize people with fire and the Holy Spirit in Matthew 3:11 and what Jesus said about being "the light of the world" in Matthew 5:13-16).
There's a scene where Markl is worried about whether they can trust Calcifer, because he's been known as a fire demon, and Sophie theorizes he may be a good kind (which is crazy, if you look at it as him beinga literal hell demon, which gave me the idea he isn't). He's not literally from the Christian idea of the devil or anything bad. He's lost, like Howl.
His fear of being looked down on, like when he wants to be seen as a powerful demon is kind of like how the Babylonian king and the devil want to be seen as great, but their power is not something a good smart person would want, the same way Sophie is annoyed with Howl for his vanity, because being good looking is desirable, even in her, but she knows it isn't what makes people love you. Not real love anyway.
Calcifer and Howl are overly vain and that destroys them, because having their power is never good enough and they always feel like they have to prove themselves. They feel complicated mess of being chained to each other but are also great friends and they don't even see that they are just like each other.
I feel like Calcifer, Howl, and Sophie are just a fantasy/symbolic look at a love triangle or how one woman affects her male love interest and his best frienemy (in this case someone he fights with, but they actually love each other), but they end up helping her too by showing her she doesn't need to feel small and that she is amazing and powerful. And the other prince is that extra guy who we all like and just feel bad for, when his love get's snubbed and he was the guy there for her all along, but he will not treat the "friend zone" like a punishment. He will cherish Sophie's friendship, even if some romantic feelings still linger.