Sebastian Michaelis is the butler of the Phantomhive household and Ciel's bodyguard, two jobs he is completely devoted to. He often describes himself with a phrase that is alternately translated as "one hell of a butler" or "a butler and a demon". 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 "contract" to abide by Ciel's will. A sign similar to the one marking Ciel's right eye can be found on his 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. He also stated that he loves cats (to the point he will get distracted from his duty to admire a tiger), and despises dogs.

His name is similar to that of Father Sebastien Michaelis, who compiled a hierarchy of demons in Hell.