Doctor (Black Butler)

"Don't you think it's normal to need the best materials in order to get the best products? As far as society's idiots are concerned, there's no such thing as success without sacrifice. A cow's bones are fine, but a human's aren't? Who decided on that?"

- Doctor

Doctor (also known as Sensei) is the secondary villain of Black Butler manga's Noah's Ark Circus arc and its third season anime. He is a medical practitioner who works at the Noah's Ark Circus to tend to the members' injuries and their prosthetic limbs, which he has constructed himself.

Appearance
Doctor is a bespectacled man with dark, unkempt hair that comes down in waves, framing his face. He consistently wears a clean suit along with a wide ribbon tied in a large bow around his neck, and black dress shoes. Doctor initially puts on a ruse that he uses a wheelchair, but he is actually able to walk.

Personality
In the beginning, Doctor appears as a kind, caring individual who has devoted his duty to provide first-aid to injured members. He also seems to have a great distaste for arguments, especially when done in front of customers. In spite of that, he is rarely ever heeded to whenever he pleads for the circus members to cease arguing, as they instead look to Joker for leadership.

However later, Doctor exhibits his sadistic nature, as well as his startling lack of regards for human life. His ambition is create the finest products, such as his prosthetic limbs, that are supplied by Baron Kelvin, who conducts gruesome circus performances to kill untrained children; their bones are the principal material that the limbs are made out of.

It is implied that Baron Kelvin has allowed Doctor to empty out the rest of the Renbon Workhouse where the first-tier children grew up, to use for the artificial limbs.

History
Baron Kelvin hired Doctor to perform plastic surgery on his face. Afterward, Kelvin served as an invaluable patron to Doctor, as he understood his ideals; Doctor desired to construct the perfect artificial limb, and thus, Kelvin sponsored him with an abundance of money and materials (i.e. children's bones).

Noah's Ark Circus arc
Doctor is examining Dagger's prosthetic limb, when Joker brings in Sebastian Michaelis, who was bitten by a tiger during a circus performance. However, Doctor is unable to find any sign of injury at all on Sebastian. Shortly thereafter, Beast enters the first-aid tent and begins to argue with Sebastian, accusing him of ruining her act. Doctor reprimands her for her failure to control the tiger when she is supposed to be the professional. He then inspects her artificial leg, which Sebastian invests great interest in. Doctor reveals that he made Beast's, Joker's and Dagger's limbs, doing everything from the carving onward. When pressed for what materials he uses, Doctor is vague, and initially says they are ceramic, but then retracts that statement, asserting that they are formed from special materials and the joints are made from doll parts. Sebastian takes a closer look at Beast's leg, and she calls him a pervert and attacks him. Doctor frantically orders Beast and Dagger to stop attacking a customer, but they ignore him. Joker notes of Sebastian's exceptional skills, and he successfully subdues both Beast and Dagger, which prompts Doctor to question why he did not do so sooner. Sebastian thanks them for their hospitality and leaves.

Sometime later, Doctor treats Ciel Phantomhive after an asthma attack. He deduces that Ciel's attack might have been actuated from catching a cold, by bathing in open air with the "resident meatheads," as he words it. When Ciel awakens, Doctor explains how asthma can be fatal, and warns Ciel to be careful. He then orders Ciel to stay in bed until his fever and cough have gone, and proceeds to kick Sebastian and Doll out of the tent.

After Ciel and Sebastian have invaded Baron Kelvin's manor, they run into Doctor in his basement. He stands up upon seeing them, which surprises Joker, who was unaware that the wheelchair was merely a prop. He then spots Kelvin bleeding on the ground, and expresses his shock, but only because Baron understood his ideals. When Sebastian urges him for more information, he states that for a long time he has been studying to make a better artificial limb, but that the materials for such were difficult to come by. He reveals that the materials are from the bones of children, and calls it "the best recycling ever." Joker is so appalled that he vomits and then questions what he saw them as, much to Doctor's dismay, as he believes no one else other than Kelvin supported his principles, as Kelvin had supplied him with materials and money. He then pulls a brainwashed, kidnapped child that Kelvin had locked up and drags her over to the sacrifice table. Maniacally, he grabs a knife and proceeds to slay her. On Ciel's order, Sebastian kills Doctor with ease.

Trivia

 * Doctor is perhaps the only pure sadistic, hateful and loveless member of the circus.
 * He is hated due to his mutilation of children, yet the murder of the girl before his demise is truly his Moral Event Horizon crossing.