Sebastian Michaelis (セバスチャン ミカエリス Sebasuchan Mikaerisu) is the butler of the Phantomhive household. He is also part of the Noah's Ark Circus, where he goes by the stage name Black (ブラック Burakku). Sebastian often describes himself through the phrase "I am simply one hell of a butler" (私はあくまで執事ですから watashi wa aku made shitsuji desukara). The phrase plays on the word "demon" (あくま akuma) and the phrase "to the end" (あくまで aku made) which can also mean "I am a demon and a butler." In the Animax dub, he says, "I am a devilishly talented butler."
Little has been revealed about Sebastian's history, except that he first came to Ciel after Ciel was about to be sacrificed in a purist ritual. Ciel ordered him to slay his abductors. This marked the beginning of their contract. It is understood that this incident occurred two to three years before the events in the main storyline (first season.) When Sebastian was summoned in the manga, he appeared in his true demon form, while in the anime, he appeared as a crow accompanied by a rain of feathers. In the manga, it seems that Sebastian was summoned accidentally, but in the anime it is made apparent that Ciel knew about the existance of demons and how to summon them all too well. Also of note, he appears in front of Ciel's attackers in the manga, while in the anime, he appears in a limbo world. In both the anime and the manga, it is implied that Sebastian will take the role of Ciel's butler until Ciel has completed his personal mission of revenge -- that is the substance of their contract. Sebastian's job is to serve Ciel, doing everything from the mundane task of serving tea, to protecting Ciel from certain death, until Ciel's mission in life is complete. In return, Sebastian will receive Ciel's soul when said mission is completed. The demon was given the name "Sebastian" by Ciel for the duration of his duties as the Phantomhive butler. This name used to belong to a dog in the Phantomhive household. He was named after the dog because Ciel was angry that Sebastian looked vaugely like Ciel's father.
Sebastian stated that, in the past, he engaged in "messily sampling every dish available," and has since resorted to only consuming "high-quality" souls. As a result, he is, apparently, starving, but states that the dinner will be that much better the hungrier he is.
Additionally, it is known that Sebastian's real home is in an alternate world, but it is quite different from the human world. In particular, he has mentioned that the creatures kept as pets there are in no way comparable to cats (the example shown resembles the Alien creature designed by H.R. Giger.) It is also said that Sebastian has been alive for at least 100 years, possibly much longer; in episode 4 of Kuroshitsuji II, Sebastian briefly mentions that he met an ancient Egyptian mummy when it was alive - the Pharaoh/mummy, identified by Sebastian as Smenkhkare, reigned about 3000 years ago, towards the end of the 18th Dynasty (1336-1334BCE), hence this hints that Sebastian would have to be at least 3000 years old to have met the man. In the anime, he also claims to be responsible for the spread of the Black Death, which occurred about 500 years before the main storyline.