The Good Doctor
The Plague Doctor is the Fear of Disease, Medicine and, obviously, Doctors. He appears as a 15th Century Plague Doctor, dressed in long, black robes, with a beak-like mask (whether or not it is actually a mask is unknown). His true gender, like all Fears, is unknown, if one even exists.
The Plague Doctor is incredibly sadistic, though in a way unlike The Slender Man, who simply will slaughter and possibly torture for a short time.
Upon encountering the Plague Doctor for the first time, one begins to develop hypocondria. A second encounter and The Plague Doctor will infect the victim with a random, slow illness.
The Plague Doctor will send a servant to care for a Runner for months, possibly years. The servant, known as an Oathbreaker, will care for and "treat" the Runner for years, slowly allowing them to suffer from the illness, giving them more and more "medical help" until the patient dies from an overdose of various medical drugs, or the said illness progresses to a point where the person is totally helpless.