@Syguy20132 In-depth critique take-two:
Looks
Ok, so, it's more than a little obvious that you have a character design that you like, because William's design shares traits with your other characters, namely being tall and the stubble. I understand liking a certain style, but it makes all of your characters seem a little samey. Do they all really need stubble? Why not a full-on beard? Or a mustache, or something? Do they all have to be tall and muscular? He's a hitman, so it makes sense on why he would be this way, but it's just something that I think you should keep in mind going forward. Also, why does he dye his hair? Does he think it'll make him look better? Or is it naturally like that because he's a demon?
Nature
His mannerisms are kinda… I don't wanna say generic because that's not the word I'm looking for. Don't most people smile when they see something they enjoy or find humorous? You said some time ago that you make your mannerisms realistic, which would be fine if you were writing a story about characters set in our nonfictional real world. But this isn't a nonfiction, and characters need to be interesting well before they're "realistic". This is a world where people can summon demons to ax people; I think a little flamboyancy is in order. If I were a demon, I would be living it up and showing off at every opportunity! I mean, who could stop me? I'm literally a demon! I think William would really benefit from being made to be more dynamic of a character. It may make him slightly less likable, but if you can pull it off, he'd be a more fun character than you initially realized.
As far as his motives, they make sense because, hey? Demons are gonna demon. But how does this fit with him being an anti-hero? After all, anti-heroes, while they go about things in ways that might make more boy scouty heroes blush, ultimately want to help people. William doesn't seem like he gives a flying [REDACTED] about anyone other than himself, and/or whoever he deems worthy of his attention, and what he finds pleasurable. He seems like he'll just kill anyone for the heck of it, and if that's true, what is stopping him from going after innocent people? He seems like a wholly evil character, and actual evil characters don't make for good heroes. If he's not wholly evil, then what are his redeeming qualities? That's one thing that most people forget when it comes to writing anti-heroes: they have to actually have parts of them that are likable. I don't like this guy (as a person, not as a character). I wouldn't trust this guy as far as I could throw him. If I needed help, what would make me, an innocent, easily-killable human, ask this guy for help?
….he really just has his whole [REDACTED] out like that? Well, I did say more flamboyancy, so I guess I can't complain. But I have to ask, why? And when? Does he ever get his comeuppance for this? Or any comeuppance at all? One major problem with characters like this is that by the story's end, they never grow and change because they don't have to. The narrative never forces them to change and they never learn from their mistakes (unless that mistake involves how to more efficiently kill people), so just keep that in mind.
His prejudices make sense for a demon, but does he ever grow past this? I know I sound like a broken record at this point, but it really needs to be repeated: what makes this guy sympathetic?
His talents are fine. I don't have to tell you to make sure that he's not too overpowered in the story, so I won't. But it would probably be beneficial for you to list what his weaknesses are somewhere on the sheet for reference.
A demon who likes to read, huh? Surely he doesn't read human literature, or else that would fly in the face of him thinking all human are useless meatbags. Unless he's a hypocrite and actually gives us credit for something, even if it's just classic literature. So what does he like to read and why? Also, can making fun of someone really be considered a hobby? Unless he goes above and beyond and tries to play physical pranks on his father on a near daily basis, I'm not so sure.
Wait, so he cares about his co-workers and animals? How does that work? What's the extent of his caring, and why those things specifically? Are his co-workers human? Or just more demons that he pals around with? If he doesn't care about his "father", then why does he listen to him? Or does he not have a choice? This is something that you really should expand upon because it's the only thing separating him from being a troubled guy who cares a little about what he's doing, and an absolute tool.
Social
All good. I like the "non-human characters enjoy human junk food" trope.
As an aside, I couldn't help but notice that your characters dislike people but like animals. You might wanna shake that up a little bit because it's a cliche not just at large, but to you personally. I know that you need an easy indicator that deep down your characters aren't all bad, but there are other cute things out there, like children. Or maybe they are particularly sympathetic towards women that are domestic violence victims, or something, anything else.
History
I'm sure you were anticipating this, but I'll say it anyway: he needs a backstory. Him just being a demon is not enough. How old is he? Did he enjoy living in Hell? What kind of demon is a "Prince of Hell"? Was he living it up and then he got ripped away and forced to do some human jerk's bidding? Or was he looking for an escape already? Did he have a love interest or a family that he left behind? All of these questions and more should be answered in his background.
All in all, William is a good villainous character, but a poor anti-hero. He needs more things about him that make him likable. And if him being unlikable was the point, then that means that you need to really flesh out his human handler, George. Who's that guy, and why did he go to such extreme measures to accomplish his goals? Or, make his co-workers so likable that William is just the token evil teammate, but that has a chance of downplaying his prominence, so keep that in mind. Here are two hospital gowns for him to cover up the boys ❤️️