Little has been revealed about Sebastian's history, except that he first came to Ciel after Ciel was about to be sacrificed in a demonic 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. 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 he dies. 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.

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. . 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.