I am simply one hell of a butler.
~ Sebastian Michaelis

Sebastian Michaelis is the antihero in the manga and anime series Kuroshitsuji. He is the butler of the Phantomhive household and acts as Ciel's bodyguard, two jobs he is completely devoted to. Sebastian often describes himself through the phrase "I am simply one hell of a butler" (わたしはあくまで執事ですから, Watashi wa akumade shitsuji desu kara). The phrase plays on the word "demon" (あくま, akuma) and the phrase "to the end" (あくまで, akumade). It can also translate to "I am a demon and a butler". 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 Faustian contract to abide by Ciel's will. A sign similar to the one marking Ciel's right eye can be found on Sebastian's 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 and was previously held by Ciel's dog. He bears a striking resemblance to Ciel's father. In the anime adaption, Sebastian can take the form of a crow. Sebastian's true demonic form has not been fully revealed. However, a depiction of sharper teeth, high-heeled boots, and elongated nails have been given. In the manga adaptation, Sebastian appears to Ciel in his true form, while in the anime version, he appears as a crow. His name is similar to that of Father Sebastien Michaelis, who compiled a hierarchy of demons in Hell.

It was later found that the type of soul Sebastian wishes to consume is one which forsakes the future, sacrifices its dreams, eludes the past, fights reality, while being stained with deep despair, and never relinquishes its nobility. Ciel Phantomhive matched all of these attributes.

After the events of the first season, he goes to Alois's manor, posing as a traveler taking refuge from the storm. His real purpose to obtain Ciel's ring, which Claude has hidden in a tin in the cellar (Claude stole Ciel's soul just before Sebastian was going to eat it). In the process of trying to protect his suitcase from Claude, it is revealed that Ciel is inside. Sebastian escapes the manor and puts the ring on Ciel's finger, telling him that it is time to wake up.

When Ciel wakes up, he has no memory of the events of the previous season, and Sebastian goes to great lengths so that he doesn't find out that their contract is over. Claude and Sebastian eventually come to a deal: their masters will hunt each other down and take revenge on each other, thus "fulfilling" both contracts. After Alois is mortally wounded, Claude decides to renig on his blood contract with Sebastian and kills Alois, as he no longer sees Alois as a fit master, so that he may pursue Ciel. He takes advantage of Ciel's confusion and brainwashes him to believe that Sebastian killed his parents, thereby causing Ciel to want Claude as his butler. Sebastian comes to his rescue and receives a final order from Ciel: that he must serve him forever as his butler, later ordering him to obtain his soul. Alois' soul possesses Ciel's body, and his wish is that neither Claude or Sebastian be able to obtain Ciel's soul, a wish Hannah grants when she turns Ciel into a demon.

Because he accepted Ciel's last order while he was human, even though he is a demon now (without a soul) Sebastian must still serve him for eternity as they both will live forever. However, this is only true in the anime.