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Sebastian takes on the appearance of a tall, handsome adult with black hair, red eyes, and pale skin. He typically dresses in a butler's outfit, which consists of black trousers, a six-button double-breasted tailcoat, and a gray vest. He has the Phantomhive crest on his shirt cuffs and tie. Sebastian also sports white gloves, a pocket watch, and a chained silver lapel pin bearing the Phantomhive crest, traditionally worn by the house's head butler.
Sebastian is a dedicated butler to Ciel Phantomhive and upholds a strong sense of duty to his master and household. He faithfully follows and carries out every one of Ciel's orders, while ensuring that the Phantomhive household runs smoothly. He does not sleep—for demons deem it as a form of luxury—and thus, he frequently performs tasks for Ciel at night. One of his quirks is that he will always respond to Ciel's demands with the saying "Yes, my lord."
Sebastian predominantly appears as a courteous and impeccable servant. However, he is, in actuality, callous, heartless, and sadistic; he is ruthless when attacking others on Ciel's orders, and had, at one point, deliberately delayed saving Ciel for his own amusement. Sebastian usually expresses his diabolic nature through his dark, sarcastic humor and inability to see good in anyone. He thinks of his own master as a brat who treats his workers rudely, but he has admitted that he does not dislike the butler lifestyle despite all the troubles he goes through on a daily basis.
Sebastian views money as "rubbish" and "materialistic." He is fluent in French and Latin, enough to teach the latter. He is also proficient in Deutsch (German) and East Franconian, a dialect in Southern Germany. Sebastian has a great and mutual distaste for Grim Reapers. Moreover, he profoundly adores cats and had kept at least thirteen hidden in his wardrobe to prevent Ciel from finding out.
Sebastian often contemplates the behaviors and attitudes of human beings. He regards humans in general to be "difficult creatures," but he is fascinated with their struggles and lofty goals. He is intrigued by how seemingly insatiable they are, for they are willing to "drag others down to get what they want, even in death."Furthermore, he is often puzzled by human behavior as demonstrated by humans’ excessive love for sweet things such as cake—personally, he doesn’t find it delicious. Finally, Sebastian has seen humans (such as Agni) bring out enormous strength from within. However, he states this is something a demon can never obtain—strength and power bred from love and faith. 
Although Sebastian is extremely interested in human behavior, there are some factors that provoke his extreme dislike—one of them being how humans are overly concerned about appearances at the most critical times. 
Sebastian's real home is in an alternate world that is quite different from the human world. In particular, he mentioned that the creatures kept as pets there are in no way comparable to cats. Sebastian had briefly remarked that he met an ancient Egyptian mummy when it was alive, identified as Smenkhkare who reigned about three thousand years ago. He was also a guest at the Schönbrunn Palace from time to time. He also claims to be responsible for the spread of the Black Death that occurred five hundred years prior. In short, he muses that Ciel Phantomhive's life in comparison would feel like an instant.
Sebastian stated that in the past, he engaged in "messily sampling every dish available," and he has since resorted to only consuming "high-quality" souls. As a result, he has been apparently starving, but he declared that the "dinner" will be much better the hungrier he is.
Three years ago, he was summoned to Ciel Phantomhive when the latter was about to be sacrificed in an occult ritual. After forming the contract with Ciel and placing the seal on his right eye, Ciel ordered him to slay his abductors. Sebastian decided to adopt a "role that is best fit to serve an Earl." Therefore, he assumed the form of a butler until Ciel completes his personal mission of revenge—that is the substance of their contract. In return, Sebastian will receive Ciel's soul. He was then given the name "Sebastian" by Ciel (which used to be his dog's name) for the duration of his duties as Ciel's butler.
Once he finished annihilating the cultists, they went to the Royal London Hospital to meet with Ciel's aunt, Angelina Dalles. There, Tanaka gave him a watch that had been passed down from one Phantomhive butler to the next. After that, Angelina arranged for a carriage to bring them to the ruins of the Phantomhive Manor. While Ciel was mourning the deaths of his parents, Sebastian reconstructed the manor.
Ciel and Sebastian then began daily life. He taught Ciel the fundamentals of being an earl; Ciel in turn taught him how a butler fit for the Phantomhive household should act. Sebastian also learned how to properly cope with Ciel's spoiled behavior; from time to time, he had to discipline Ciel. He later attended Ciel's coronation as the Earl of Phantomhive, but he did not observe at a close distance because he saw himself as a mere butler, not family. While they were walking, Sebastian asked Ciel if he was happy with how things were. Seeing Ciel's hesitation, he attempted to devour him. He stopped, however, when Ciel suddenly turned around and stated that he was determined to attain his revenge. He commanded Sebastian to be his sword and shield and bring him victory. Sebastian complied, as he was sure that Ciel's soul would be extremely delicious.
Sometime during the same year, Sebastian was summoned to Ciel's room. Ciel complained of a wobbly tooth that was preventing him from eating his food properly. After examining it, Sebastian decided to forcibly take it out, much to Ciel's chagrin. Ciel attempted to protest against the idea, but Sebastian proceeded regardless, under the impression that he was doing his young master a favor. After the tooth removal, Ciel threw an angry fit at Sebastian. Ever since, he has been unwilling to allow Sebastian to check for cavities in his teeth when they start to wobble.
- (To Ciel Phantomhive) "Yes, My Lord."
- (To Ciel Phantomhive) "It seems your talent for dancing is non-existent. It must be crushing, young master."
- "I am the butler of the Phantomhive family. It goes without saying that I can manage something like this."
- "I am one hell of a butler. So long as the young master possesses the 'mark of the covenant' . . . I am his faithful dog. A 'sacrifice' . . . a 'wish' . . . and a 'covenant' bind me to my master.
- "Young master. Forgive me. As butler of the Phantomhive Family, this is an unpardonable disgrace . . . I do not know how I should atone for this . . . I have been unable to prepare dinner this evening."
- "A butler begins his day early. He is the last to finish his work late at night, and the first to begin work in the morning. Such is the duty of a butler who manages the household."
- (Referring to Mey-Rin) "Why, this imbec—that is to say, housemaid. It would seem the problem no longer lies in her eyesight. Why does she fail to realize that there is something clearly wrong with her own brain and not the instructions? Word has it that the public adores silly maids. But I do not think I will ever understand it, and I have no intention of even trying to do so. I myself would like to wring her neck."
- (Referring to Baldroy) "Why this imbec—rather chef. You should talk about cooking after you have actually cooked something. I believe eighty percent of what you have managed to cook has been charcoal. The other twenty percent was hazardous waste. Save your art for you hairstyle, and cook something edible . . . otherwise . . . I wish for you to become charcoal yourself."
- (Referring to Finnian) "Why this imbec—this imbecile! How can a gardener be so clumsy? Though the saying goes, 'an idiot and a pair of scissors can both be of use,' giving this useless idiot a pair of scissors is trouble in itself. He refers to something that happened two or three days ago as being 'a while back.' The way his brain can easily cast aside a blunder of that degree . . . I have surpassed anger, and I am, in fact rather impressed."
- (About Finnian) "I have lived for quite some time, but this is my first encounter with a being from outer space. How does he expect me to respond to his beaming smile?"
- "I have had enough. In times like these, yes . . . I want to see her. A supple body, flowing black hair. I must hurry . . . to the place where she awaits me . . . to her. I find cats most agreeable. They only say (do) that which is necessary. They are quite adorable. They do not exist in my world. Pets do exist over there, but . . . they leave much to be desired."
- "I, your 'chessman,' am your 'might,' your 'hands' and 'feet' . . . you alone will decide . . . you yourself will choose . . . and for that, I shall become your 'might.' You said as much . . . 'that day.' I am merely a 'butler.' I will not cross that line. I only faithfully execute what the master orders, and answer what the master asks me. By your order . . . I, your 'chessman' . . . shall become your 'sword.' Now then . . . let us move into check, Master."
- (To Grell Sutcliff) "From the time . . . my master called me 'Sebastian' . . . that word became my covenant and christened me anew. I have been 'Sebastian' since that day. I vowed as much to the moon, you see?"
- (While stepping on Grell Sutcliff's face) "I do not enjoy being stepped on . . . but being the one doing the stepping is another story altogether."
- "Humans are easily tempted. When they are poised on the edge of hellish despair . . . and a spider's thread of salvation presents itself, they will invariably grasp it . . . no matter the manner of human."
- (To Frances Midford, about Ciel Phantomhive) "The young master has an inordinate talent for games. Consequently, he seems to over-confidently believe that he 'cannot lose.' But at times, it will be necessary for him . . . to carry himself with humility while striving for his goal. Otherwise, he will eventually trip up . . . as the place for which young master aims is not an easy one to reach. Perhaps it is not my place to say this, but . . . I would like you to be an exemplar for my master, Marchioness."
- (To the other Phantomhive servants) "Why do you three not kowtow and beg Mister Agni for the dirt under his nails? Your lives might gain a little value that way." 
- (To Ciel Phantomhive) "—Oh? What a tiny master this. You have summoned me. That fact will not change for all eternity . . . and the sacrifice that has been made will never return. Make your choice."
- (To Agni) "England or India, it makes no difference. No matter where or from what era, they are all alike . . . those beings called humans."
- (To William T. Spears) "I grew weary of indiscriminately devouring souls long ago. The more ravenous I am . . . the more delicious the dinner."
- (To William T. Spears) "My goodness. That must be quite a chore to handle alone. Still, a large number of souls . . . shall I lend you a hand?"
- (To Ciel Phantomhive) "As long as there is the contract, I will protect your life. However, if you make a mistake of your own accord, you will possibly suffer a painful experience while alive. You already know that, don't you? Any game becomes boring when it lacks thrills."
- (To Beast) "Why not forget just for the tonight? Drown in pleasure . . . and breathe sweet poison deep into your lungs."
- (To Ciel Phantomhive) "You are different from demons who are revolting and possess complex evil intent. Telling lies . . . desperately struggling, kicking down other people. Stealing the stolen, while repeating your reasons over and over. And even so, you aim for the horizon over the hills. That's why humans are interesting."
- (To Elizabeth Midford, about Grell Sutcliff) "He's just a pervert. I'm afraid it might be contagious, so please step back and stay away from him."
- (To Ronald Knox) "Young’uns these days are really so feeble. Isn't it kind of old-fashioned to just rely on your Death Scythe?"
- "How interesting... Humans will drag others down in order to get what they want even in death. What truly insatiable creatures!"
- (To Ciel Phantomhive) "Society has monsters lurking that are far nastier than a demon like me."
- (Referring to Ciel Phantomhive) "He turns his back towards the light without hesitation. That figure that charges into the abyss at a dignified pace. How beautiful and foolish."
- (To Undertaker) "That's where we differ. Exactly because 'death' is such a hopeless and definite 'end' . . . it is beautiful."
- (About Ciel Phantomhive) "Ah, my master. You're misunderstanding something. What I find most entertaining now . . . is playing the butler in my game with you. It's not behaving like a frenzied beast. However, if that's how you like me . . . I'll act accordingly. Because I'm one hell of a butler."
- (To Ciel Phantomhive) "The candle burns away to keep its flame alive — not unlike a human being. An inconstant flame that flickers dangerously in a gust of wind and is extinguished all too soon. But, that is what makes them all the more beautiful."
- "If it's your wish I will follow you everywhere. Even if your throne crumbles and your shiny crown turns to rust. Beside you as you lie, softly down, I will be. Until I hear the words, 'checkmate'."
- (To Ciel Phantomhive) "For a king to survive it's good for him to be able to skillfully manipulate the other pieces using both the knight and the queen. Beneath the throne the bodies of your pieces will pile up along with your sins. You must not lose."
- (To Claude Faustus) "Demons have eternal life. We've grown weary of living so long . . . far too long. My young master's presence lends spice to the world we're glutted on."
- (About Queen Victoria) "As expected of her majesty—even her selfishness is at a royal level." 
- (Thinking about Ciel Phantomhive's orders) "Ohh — dear master. I am afraid you have the wrong impression. My greatest amusement at the moment, you see, is the game I, while bound by my butler livery, am playing against you. So it would not do to act on impulse and behave like a beast. However, if you are particular to my being such a devil . . . I shall act accordingly. For I am a devil of a butler." 
- (To Ciel Phantomhive) "If you disappear, I will be most distressed and worried. Because if my master is not here, I wouldn’t be able to be your butler."
- (To Edward Midford) "I have seen a great many churches and temples laid to waste in religious wars, all in the name of shaking the faith of the believers. But they were nothing more than symbols. Even if they were decimated, the effect was only temporary. On the contrary, their ruin could even intensify the zeal of the people. The more emotions are oppressed, the more they tend to flare up. That is the kind of creature man is, do you not agree?"
- "Human hearts are quite irksome, you see. Unlike the flesh, you cannot see or touch a human heart. And not even a devil or god can bind the heart of another in the truest sense of the word."
- According to the official Kuroshitsuji's character popularity poll, Sebastian is the most popular character in the series, with 4014 votes. He is ranked among the "Devil Six."
- Sebastian's name is similar to Father Sebastien Michaelis, a French inquistor who wrote a classification of demons of Hell from the demon Berith while exorcising a nun.
- On Sebastian's grave, his epitaph included the following inscriptions: 'To The Memory of Sebastian Michaelis. Died March 1889. May ye be in heaven an hour before the devil knows you're dead."
- In the Victorian era, butlers were called by their last name; Sebastian should technically be called Michaelis by Ciel and Mr. Michaelis by the other servants.
- Whenever Sebastian was summoned in the manga, he appeared in his true demon form; in the anime, he appears as a crow or raven accompanied by a rain of feathers.
- In early drafts, Sebastian's hair was styled with a 7:3 ratio parting. But because Yana Toboso's editorial staff and colleagues did not like the hairstyle, she had to move his parting into the middle.
- Sebastian's last name originates from the Japanese word mikairi meaning "repayment," signifying his contract with Ciel. 
- In accordance with his “aesthetics/bigaku”, Sebastian likes to hear stories about Ciel’s past. 
- Sebastian uses his powers to get things done faster than any normal human could. Some examples are as follows:
- He can make Iris flowers grow from seeds to full bloom within a day. 
- He has the power to “will” his hair short. 
- He can obtain all kinds of information from various sources effortlessly and efficiently, as he did during the Red Butler Arc.
- In the Indian Butler Arc, he tasted all kinds of spices to create various optimal blends; he also used his superior sense of smell to pick the right ones. 
- He can “pick up” signs of human life by sensing their souls from a fixed range. 
- His “sense of taste” is different from humans. 
- He can make Iris flowers grow from seeds to full bloom within a day. 
|Ciel Phantomhive · Vincent Phantomhive · Rachel Phantomhive · Sebastian Michaelis · Tanaka · Baldroy · Mey-Rin · Finnian · Pluto · Snake|
|Baron Kelvin · Joker · Beast · Dagger · Wendy · Peter · Jumbo · Snake · Doll · Doctor · Suit · Smile · Black|
|Sebastian Michaelis · Claude Faustus · Hannah Annafellows · Cantebury · Thompson · Timber|
|The headmaster • Johann Agares • Mr. Michaelis|
|Edgar Redmond • Gregory Violet • Herman Greenhill • Lawrence Bluewer|
|Cheslock • Clayton • Edward Midford • Joanne Harcourt • Maurice Cole • Derrick Arden|
|Ciel Phantomhive • Derrick Arden • McMillan • Soma Asman Kadar|