Sebastian is a devoted butler Ciel Phantomhew, who adheres to a strong sense of duty to his master and his family. He correctly follows him and fulfills each order of Ciel, ensuring that in the house of Phantomhew everything passes smoothly. He never sleeps, because watching sleep for a demon is a luxury, and thus often performs Seil's tasks at night. One of his remarkable features is that he always responds to each order or demand of Ciel with the phrase: "Yes, my lord."
Sebastian is a devoted butler Ciel Phantomhew, who adheres to a strong sense of duty to his master and his family. He correctly follows him and fulfills each order of Ciel, ensuring that in the house of Phantomhew everything passes smoothly. He never sleeps, because watching sleep for a demon is a luxury, and thus often performs Seil's tasks at night. One of his remarkable features is that he always responds to each order or demand of Ciel with the phrase: "Yes, my lord."

Despite the fact that externally Sebastian behaves as a well -mannered and impeccable butler, in reality, he is a callous and heartless sadist. He demonstrates these features in a very passive and sarcastic manner. He is ruthless in battles with those whom Ciel orders to attack: for example, he beats Grell Sutcliffe in the face during their battle, because he cherishes the most with his face. His devilish nature is also shown through black, sarcastic humor and inability to see something good in someone. The foam, Sebastian behaves indifferently, and even deliberately delays the salvation of Siel for his own entertainment for his own entertainment. Despite all the troubles through which he passes daily, he says that the way of life is not as bad as it might seem.

Sebastian sees in money only “garbage produced by people”, and can calmly give the pocket money that is issued to him. He speaks freely in French and Latin, which allows him to teach these languages ​​of others. Also, he speaks the German and East Francon, the dialect of Southern Germany.Sebastian has a strong and mutual hostility towards the reapers. The other side, he shows great interest in cats, and holds at least thirteen in his closet, hiding them there from Ciel.

Sebastian often reflects on the behavior and relationships of people. He considers people as a whole “difficult creatures”, but adds that the struggle and high goals make them interesting. He is also intrigued by how they are insatiable, because they are ready to "drag another to the same bottom, just to get what they want, even after death." It is often puzzled by how much people can like sweets, for example, a cake, which herself personally does not find delicious. In conclusion, Sebastian is able to see people who are hiding enormous power (such as Agni), also noting that demons are not able to get it, since such a force can be born only from love and faith.

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. Furthermore, Sebastian existed for a rather long period of time, and therefore, he occasionally 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.

In January 1886, Sebastian was summoned to Ciel Phantomhive in a cult's summoning ritual at a cathedral. After convincing Ciel to form a contract with him and placing the seal on Ciel's right eye, Ciel ordered him to slay the cultists. 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.

After burning down the cathedral on Ciel's order, Sebastian accompanied Ciel 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 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. ter 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 the added pain that would come with the road to Ciel's revenge would make his soul "extremely delicious."

In 1887, 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.

Sebastian endeavored to recruit servants for Ciel. He approached Baldroy, a soldier fighting in a war in America, in a cave.