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.



It is also indicated that Sebastian has been alive for at least one hundred years, possibly much longer; Sebastian has briefly mentioned that he met an ancient Egyptian mummy when it was alive, identified by Sebastian as Smenkhkare who reigned about three thousand years ago. He has also claimed to be responsible for the spread of the Black Death that occurred five hundred years prior. In short, he muses that Ciel's life in comparison would feel like an instant.


Sebastian states 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 claims that the "dinner" will be that much better the hungrier he is.



Three years ago, he was summoned to Ciel Phantomhive after the latter was about to be sacrificed in an occult ritual. After forming the contract with Ciel and placing the seal of the contract on his eye, Ciel orders him to slay his abductors. 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 goal is complete. In return, Sebastian will receive Ciel's soul when said goal is completed. Sebastian is then given the name "Sebastian" by Ciel for the duration of his duties as Ciel's butler.



After killing the cultist that tried to sacrifice Ciel, they went to the Royal London Hospital to meet with Ciel's aunt, Angelina Durless. There, he meets Tanaka who gives 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 his parents, Sebastian reconstructs the manor.




Ciel and Sebastian then begin daily life. He teaches Ciel the fundamentals of an Earl, and Ciel teaches him how to be a butler. He later attends Ciel's coronation as the Earl of Phantomhive.

Appearance

Sebastian takes on the appearance of a tall, male adult with black hair and red eyes. He typically dresses in an outfit that consists of black trousers, a tailcoat, and a vest. He has the Phantomhive Crest on his shirt cuffs and on his tie. He also sports a pocket watch with a chain, and a chained silver lapel pin bearing the Phantomhive Crest, traditionally worn by the house's head butler.
His white gloves conceal the mark of his Faustian Contract with Ciel Phantomhive on his left hand. He occasionally removes these, revealing black fingernails.

Sebastian's demonic form has never been fully shown. Details of this form include high-heeled stiletto boots, a shower of black feathers, sharp claws, glowing fuchsia irises, slitted pupils, and longer, sharper teeth.Prior to transforming, Sebastian says several times that his form is unpleasant to look at. Just looking at it drove Ash Landers to hysterics, screaming that he was "unclean".




Personality

Sebastian is a dutiful and dedicated butler to Ciel Phantomhive. He follows each order faithfully, while always ensuring that the Phantomhive house runs smoothly. However, whenever Sebastian sees a cat, he ignores whatever Ciel says to him. As such, he is noticeably annoyed whenever they end up behind schedule. The way he responds to Ciel's demands is quite notable, as the servants are pleased when they can successfully imitate the way he says "Yes, my lord" to Ciel.
Despite his ability to appear like a perfectly normal human, especially one who is able to smile, be cheerful, and be understanding, Sebastian is, in actuality, quite heartless, sadistic, and malicious. This comes out when he attacks others on Ciel's orders, such as when he kicked Grell Sutcliff's face because it was the thing that Grell wanted the least damage to. It is also shown with his cruel sarcasm and his inability to see good in much of anyone. At times, Sebastian can be indifferent and deliberately delays Ciel's wellbeing, as he frequently teases him and has put his life in mild danger for his own amusement. Once, he responded to Ciel's kidnapping as "terrible" but only because the tea he prepared would go to waste. He also calls Ciel a "brat" who treats his workers rudely. However, despite all the troubles he goes through on a daily basis, he has said that he doesn't think it's such a bad way of living.

Sebastian often contemplates about the nature of human beings. He seems to find humans, in general, to be "difficult creatures," but he also states that their struggles and lofty goals make them "interesting." He is also intrigued about how insatiable they are as they are willing to "drag others down to get what they want, even in death".