Sebastian Michaelis is the butler of the Phantomhive household and Ciel's bodyguard, two jobs he is completely devoted to. He often describes himself with a phrase that is alternately translated as "one hell of a butler" or "a butler and a demon". He is highly skilled in any area 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 as the butler to the Phantomhive family. This is due to the fact he is a demon bound by a "contract" to abide by Ciel's will. A sign similar to the one marking Ciel's right eye can be found on his left hand, which has been used on one occasion to twist the arm of a kidnapper without touching him. The name Sebastian is in fact a temporary name given to him by Ciel. He also stated that he loves cats (to the point he will get distracted from his duty to admire a tiger), and despises dogs.
His name is similar to that of Father Sebastien Michaelis, who compiled a hierarchy of demons in Hell.
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 just one hell of a butler" and "You see if I couldn't kill a man, what kind of a butler would I be?" (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 simply one hell of a butler" which is, obviously, another play on his demonic origins.
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, subliminally. 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. Sebastian is also shown to be obsessed with cats, kittens, wild catlike beasts, and so on. 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. 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. In the anime adaption, Sebastian's demonic form is associated with a crow or a raven. Sebastian's true demonic form has not been fully revealed in the manga or the anime. 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..
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. 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 then obtains Ciel. But due to Alois's wish which for neither demon to be able to obtain Ciel's soul, Hannah in her last moment lets Sebastian know that Ciel is no longer human but a demon. As per his contract, on never actually having devoured Ciel's soul, he was ordered to be Ciel's butler for eternity.