Sebastian Michaelis (セバスチャン・ミカエリス Sebasuchan Mikaerisu?) is the butler of the Phantomhive household and acts as Ciel's bodyguard. Sebastian often describes himself through his catchphrase "I am simply one hell of a butler" (私はあくまで執事ですから Watashi wa akuma de shitsuji desu kara?). The phrase plays on the word "demon" (あくま akuma?) and the phrase "to the end" (あくまで aku made?). It can also translate to "I am a demon and a butler". In Funimation's English translation, this phrase becomes "I am a devilishly talented butler" or "I am one hell of a butler" which is, obviously, another play on his demonic origins.
He has been described by multiple women (Madame Red, city women, nun in anime) throughout the anime and manga series to be extremely attractive and seductive.
Being a butler is a job he is completely devoted to. He follows Ciel's orders faithfully, however, he does not seem to be overly concerned about Ciel's well-being most of the time, as he insults and irritates Ciel to see his reaction. However, occasionally he does appear to care about Ciel at times, such as in the Noah's Arc Circus arc where he was worried when Ciel had an asthma attack, and again later on when Ciel had a panic attack. He calls Ciel a "brat" who treats his workers rudely. Sebastian is also shown to be obsessed with cats, kittens, wild catlike beasts, and so on; in chapter 44 of the manga, he is shown to be hiding 15 cats in his room. In chapter 49, it is shown that he had found the cats outside in the rain when he was doing a task for Ciel, and he just couldn't leave them outside. He also appears to love a stray black cat with amber eyes, who occasionally appears near the Phantomhive household or inside the household's garden, waiting for him. Comically, the only times he does not follow Ciel's orders are when he sees a cat, too preoccupied with it to pay any attention to anything else. This occurred in the second season of the anime when he sees a mother cat with her kittens, and Ciel calls him a "cat obsessed moron." Also, he doesn't appear to hear anything that Ciel says when he is looking at a black cat in the alley they are hiding in right before Mary Jane Kelly is murdered. He has an extreme aversion to dogs, and it was much to his dismay when Pluto the hell hound came to stay at the manor.
He is highly skilled at practically everything, and handles every task, no matter how impossible it is, as well as any problem that arises from the errors of the other staff with ease, stating it is only natural the butler of the Phantomhive household is capable of such. This is due to the fact he is a demon who is bound by a Faustian contract to abide by Ciel's will. A sign matching the one marking Ciel's right eye can be found on Sebastian's left hand. The mark of the contract has been said to allow Sebastian to find Ciel. The name Sebastian is, in fact, a temporary name given to him by Ciel and was previously held by Ciel's dog. It has been established that he is a cat-person, ignoring Ciel's words to comfort a stray cat. In the anime, he also shows high disdain for dogs, especially towards Pluto. In the anime adaption, Sebastian can take the form of a bird similar to a crow or a raven. In the manga, Sebastian does not appear in this form. Sebastian's true demonic form has not been fully revealed in the manga or the anime. However, a depiction of sharper teeth, high-heeled boots, and elongated nails have been given in both. In the anime, his true form is also shown, but only briefly in glimpses at the finale, sporting high-heel boots and claw-like hands. In the manga chapter 19, a similar appearance has been shown. In the manga adaptation, Sebastian appears to Ciel to form the contract in his true form, while in the anime version, he appears as a crow. He is voiced by Toshiyuki Morikawa on the first drama CD, Daisuke Ono in the anime adaption and on the second drama CD, and J. Michael Tatum in the English dub. His name is similar to that of FatherSebastien Michaelis, who compiled a hierarchy of demons in Hell.
In The Second TV Series, Sebastian resurrects Ciel with his Family Ring (which was somehow located in a "New Moon Drop" tea box under Alois Trancy's manor). He comes into Trancy's manor in the disguise of a wanderer needing to shelter himself from a storm, however he criticizes Claude Faustus's abilities as a butler saying he does not seem to take his responsibilities seriously when he sees a drop of sauce on the side of his dinner plate. This alerts Claude that he is Sebastian as he attacks him later on when he gets the "New Moon Drop" tea box from Alois. In the battle, the large traveler's trunk he is carrying opens, revealing Ciel's lifeless body. When Alois gets excited, seemingly recognizing Ciel, and wants to touch him, Sebastian says that he cannot allow such a low person as Alois and his copy-cat butler Claude to catch them. Sebastian resurrects Ciel and follows his orders obediently as before, however, he does not mention the Trancy incident or tell Ciel anything that has happened (it seems Ciel has forgotten a lot of things that had transpired in the first TV series). The air around the relationship between Ciel, Sebastian, Alois, and Claude is an enigma that seems to be one of the pinnacles of the second series.
It is later revealed that when Sebastian had wanted to consume Ciel's soul, Claude came (in the form of a spider) and stole Ciel's soul prior to having Sebastian consume him - thus voiding their contract. Claude then merely tells Sebastian that he was acting upon the orders of Alois Trancy who wanted to obtain Ciel. As Ciel has forgotten the revelations at the end of season one, his soul is incomplete. Sebastian and Claude then forge a contract with their blood upon a rose (which turns black) that Claude will only come after Ciel when Ciel's revenge is complete and Sebastian agrees that Alois Trancy will be the target of Ciel's revenge. But the progress of this contract has gone haywire. Claude (in episode eight) kills Jim Macken (aka Alois Trancy) because Jim has expressed love for him - this makes Claude disinterested in consuming Alois' soul. He hypnotizes Ciel and puts the Trancy ring upon him (which contains Jim's soul) and makes him believe that Sebastian had killed his parents (as Alois and Ciel share similarities with their memories being mixed). Sebastian becomes helpless when Ciel, using his contract's seal, tells him to disappear from his sight as he now recognizes Claude as his only butler and that Sebastian was responsible for the deaths of his parents and the burning of the Phantomhive household.
Sebastian complies only to go away to a distance and mutters that though he was late in preparing dinner, it will be served - implying that he will defeat his enemies. His form becomes glowering black and eyes glow red, for there are no longer any 'contractual handcuffs' to obey and claims that he will, "Clean up every last spider."
He later defeats Claude, Hannah, and Alois, and obtains Ciel, though due to Alois's wish (for neither demon to be able to obtain Ciel's soul), Ciel turns into a demon. Since Ciel had ordered Sebastian to be his butler until the day he took his soul, at the end of the second season, Sebastian would have to remain as Ciel's butler for eternity.