So, first of all, it's worth saying that we know very little about how demons work in Kuroshitsuji. The manga provides almost nothing in terms of elaboration, and the only information we have about demons other than Sebastian comes from Kuroshitsuji II, which is now dubiously canon. But hey, I'll take what I can get...
general appearance | true form | demonic abilities | spiritual permissions | butlering
Sebastian is a conventionally attractive man in appearance, and is supposed to be a walking example of tall, dark, and handsome. He's almost always dressed in black, and pays quite a bit of attention to his appearance, but doesn't give off the impression of being vain so much as particular. In terms of build, Sebastian is lean, but muscular. Normally, Sebastian's eyes are an amber color with a reddish tint, but not quite enough to be completely unnatural, as seen here. If you refer to his eyes as being red, don't worry, I'll go with it! Canon isn't consistent on this, so my icons reflect red eyes, I know...
When Sebastian is using a bit more of his demonic abilities, his eyes are the first to change, turning a much more vivid, blood-red and his pupils will dilate into slits, as seen here. Characters may also notice that his teeth are much more vicious looking as well, though it's also worth noting that his first premolar is always noticeably sharper (seen here), but Sebastian's smiles are so clipped that this usually isn't visible.
The fancy butler look that Sebastian dons is not his true form, and Sebastian implies* that he takes on forms that suit his contracts best. So in this case, to serve an Earl, a handsome, human butler is best. This form is not an illusion, being very much solid and able to bleed, but it's still something of a facade for Sebastian's true form.
This is where we get to the fabulous heels you see above. Sebastian's true form has never been seen in full, and is pretty much limited to this character design sheet in terms of what we have seen. So, yes, he has truly fabulous heels and a hand that looks like it's covered by some kind of carapace. In the first anime series, Sebastian's true form is often obscured by a multitude of feathers, but in the manga (which is the one I will use), it takes the form of much more ethereal shadows, which Sebastian is able to control and are able to grip and pick up solid, even heavy objects. These shadows can be "released" in varying quantities depending on what Sebastian needs, and are also quite frightening to animals. You can see other examples of the shadows here: one, two, three.
It's also worth noting that Sebastian is very heavily associated with crows, hence the black feathers used in the anime. The connection has never been explicitly stated in canon, but because of the idea that Claude Faustus, another demon that shows up exclusively in Kuroshitsuji II, is a spider demon/is actually a spider, I play Sebastian with the same idea extending to crows. As such, my personal headcanon for Sebastian's true form is much closer to this fanart.
IC, Sebastian is unlikely to bring this connection up, but we'll say this is just a "for your reference" thing. For characters that are familiar with demons and demonology, if they were to learn about this connection, they might come to the conclusion that Sebastian is Malphas, though he'll neither confirm nor deny, and OOC, I won't either! It's something I'd prefer to leave vague. Similarly, I also leave Sebastian's connections or lack thereof to religious frameworks vague, since the canon seems to be purposefully vague itself. Sebastian never speaks of being from Hell, but rather, "a realm separate from humans," though it's perfectly reasonable for a character to assume that he means Hell.
This is a lot about what Sebastian is, and not so much about what he can do, so now we'll get into that. First of all, this section is limited to those skills that are demonic in nature, so pretty much his feats of strength/speed/etc. Skills that are more in line with being a butler are covered in a later section.
To start with the basics, Sebastian is extremely physically capable. In general, you can expect Sebastian to have abilities that are quite enhanced as compared to a human, and in all examples provided, these feats are done without much effort, which implies that these feats are all well within Sebastian's abilities.
First of all, Sebastian is quite physically powerful, from picking up large objects to crushing skulls with his bare hands. He's also extremely swift and acrobatic, being able to run long distances very quickly, jump extremely high, and also just show off. He's especially fond of the last use. Sebastian's senses are all also enhanced, with two exceptions. His sense of taste is quite different from humans', such as chocolate tasting foul, though with Ciel's rather rude and harsh tutelage, he's able to estimate what tastes work well together objectively. The other exception is an additional sense, where Sebastian is able to sense the souls of living beings. This is very vaguely defined, and as such, I will not use it without someone expressing interest first and our talking about it, because whether a character has a soul or not isn't my call to make.
Otherwise, Sebastian's endurance and durability are also quite extreme. Even after a severe injury, Sebastian is able to fend off zombies singlehandedly and with a boat oar for about twelve hours straight, as an example of his endurance. Human weapons only have superficial effects on him, where he's able to heal them instantly at his will, which even extends to things like mustard gas, which does affect him, but he's also able will away. He also doesn't have limitations of a human body, such as being able to inhale that mustard gas and hold it within himself without ill effect, and able to handle freezing water without so much as shivering. He's also able to manipulate fire to some degree, while being completely unburned by it himself.
So what about weaknesses? Well, in strict canon, they're very limited. The only weakness of Sebastian that we know of is the Grim Reaper's death scythes. These weapons are able to legitimately injure Sebastian, whereas human weapons are not able to inflict permanent damage, as explained above. However, even in this case, they're not on the same level of severity as it would be for a human, since Sebastian gets a scythe through his chest, and yet is able to exert himself to fight zombies for about twelve hours afterward, which is the only time we see Sebastian exert himself so far.
However, this is of course not fun or fair for RP, especially since there are many, many types of abilities that aren't represented in Kuroshitsuji, but are well represented in RP. In general, talk to me! I'm very flexible and willing to work with characters' abilities to make fights fun and fair, and I'm often inclined to just make Sebastian lose since he is such a stupid godmod and can take the loss much more easily than most characters. You're also welcome to ask about that on this post!
That said, one category in particular that's worth addressing is holy powers. While angels do exist in anime canon (for S1 + S2), I consider that plot point very much noncanon since it contradicts some elements in the manga. I play Sebastian's connection to religious frameworks as being vague to reflect the fact that it's also vague in the manga, but holy powers will have an effect on him. It works similarly to how the Death Scythes work in that an offensive attack will genuinely injure him, though also like the Death Scythes, it's still rather difficult to actually kill him with it. The exception is latent uses of holy powers, such as blessing a place so that Sebastian can't enter, as a random example. These will not work (since Sebastian is able to enter churches/hold crosses without problem) though if you let me know, I'll definitely work with you on these! In the past, I've played these effects as if Sebastian is having an allergic reaction, so I'm definitely flexible.
For characters that are able to sense the supernatural, there shouldn't be anything stopping them from being able to sense that Sebastian isn't human or is a demon, depending on your character's level of sensitivity! Sebastian has no such sense himself, so it won't go both ways. However, before a character comes to this conclusion, I'd like to be given a heads up for OOC reasons—Most of the time, I'll be fine with it, but sometimes I may ask that you hold off, especially if Sebastian is new to a game. The exception is CDC, in which case, go for it, since it's on his IC File anyways. If your character has a more specific ability, such as being able to sense souls/life energy, hit me up! There's a special permissions post for this ability here.
Besides being a demon, Sebastian is simply one hell of a butler, of course. This is actually a new venture for Sebastian relative to his whole life, since many of the skills he has have only been learned since he started working for Ciel. Despite that, Sebastian is a nearly perfect butler, and performs all of his work excellently, which comes down to his own personality more than anything. Sebastian is a perfectionist and a hard worker, so he strives for perfection because Ciel demands it, but he's pleased by it just as much.
On that note, despite being a demon and seeming to be able to do things swiftly and correctly with no problem, Sebastian does learn things like a normal person! He's shown studying while Ciel sleeps, and when faced with something he doesn't know about, he researches it. Sebastian does this without complaint because he seems to enjoy studying and learning, even showing an enthusiasm for technology. He's a nerd, basically.
He's able to learn things very quickly because his memory as a demon is eidetic, so when he sees something once, he's able to do it extremely well himself. The exception to this is creative endeavors, since Sebastian isn't creative at all. He may seem to be, since he's able to create beautiful things like flower arrangements, but these are all logical imitations and evaluations based on his understanding of how humans appreciate aesthetics. Again, nerd.
Otherwise, Sebastian is extremely devoted to his duties as a butler. While Ciel may demand it, Sebastian never does his work begrudgingly, even when doing humiliating things. From dressing up as a unicorn to digging in a landfill, he does all his work as needed. He seems to enjoy menial chores the most, though, such as laundry and cooking, and Sebastian truly excels at all tasks of housekeeping, even referring to it as a "butler's aesthetic." The most notable of his butlering skills is his cooking, which despite his odd sense of taste as a demon is always nothing less than gourmet.
On one other odd note, Sebastian is also an especially good teacher. He's quite harsh as a teacher, but if you wish to learn something, Sebastian will absolutely make sure you do. Ciel gets his lessons with rude sarcasm, but when Sebastian masqueraded as a teacher, it was noted that he was especially thoughtful to students and a good teacher overall. Go figure.
This is very long and unwieldy, I know, and I apologize for how stupidly hax Sebastian is... One of the main points of his character is how (seemingly) perfect he is, where his nasty personality is a balance to his hypercompetence in very nearly everything. Though of course, this isn't always clear in RP or fun to RP with! If you ever want to know more about Sebastian's abilities, want to ask how your character's abilities might work in relation to his, or anything else, this post is always open for questions or concerns! Hit me up, friends.