William is the main character of the story. He's a very smart boy who is the heir of the rich Twining family, since his parents died when he was too little; anyway is his uncle, Lord Barton, who usually takes care of any issue and of him. As I wrote above, William is very intelligent and he loves to study. His goal is to become a politician or a lawyer, in short, he wants to be part of the élite. He's the best student of his school, and with his marks, I bet he could do everything he would, but he has a little problem: a teacher informs him of the fact that his fees had not been paid. It would really be a problem for him and for his future dreams... when he comes back home during holidays, he finds out, thanks to Kevin (his butler), that his family went bankrupt and that his uncle disappeared. His home (empty) is the only thing left, aside from Kevin that's the only servant who decided to remain there because he and his family were servant from a long time.
The only thing that William wants to obtain is the money for his school fees, nothing more. So he tries to search all over the house, and he can't find anything but a demon that he summons by mistake, Dantalion. Since William is a realist and doesn't believe in such things, he will wait until he learns if the demon has something of a certain value with him or not, then he will hand it over the police. He will, anyway, learn from him, that he is in some ways related to Solomon, the Emperor of the Demon World... he is his descendant. But, as expected from a person as William, he won't ever believe to such a thing. He's a boy who thinks that science is the only way that can explain things in life, magic, demons and such dont exist, neither gods, in fact, he usually doesn't follow with passion religion's classes, because for him, God does not exists either. When he will be in trouble and Dantalion will save him against Gilles the Rais, he will be a little catch unprepared, but he will never admit that these creatures can exist.
In his "private" life, anyway, he's a very calm,
lording over the others without let them understand XD, friendly and... he usually like to stay on his own. He's the president of the student council. He doesn't have a lot of friends, I can say that he has any friend at all. But he's very kind with everyone and always tries to do his best to be helpful. Anyway, he's often with both Swallow and Isaac, but they're more annoying than other to him even if at the end, he always decides to do the best for them and help as he can.. The only person he esteems is his butler Kevin, that for him is more than a friend: in fact William is himself just with him. He's the only person who really took care of him since he was little, so, little by little, he gained his trust. We can see that his personality changes when it comes to a demon or a supernatural creature: William starts to go off his rocker and to become intractable, even because he doesn't even want to listen what people want to tell him (and they do it anyway XD). Even if at the begin he could be unwilling, he's always there to help anyone who needs a hand, both if it is a human or another creature.