Contractors are individuals who have gained superhuman powers due to the appearance of the Hell's Gate and Heaven's Gate. Known Contractors have a very wide array of powers, and each contractor appears to have a power unique to him or herself. When a Contractor's powers are used, his or her eyes glow red.
The existence of Contractors is kept a secret from the general public by most governments around the world, but they are well-known to the higher echelons of society and in the criminal underworld. Contractors possess two other traits that give them a rather sinister reputation: the need to perform Obeisance, which is a bizarre, obsessive compulsive "payment" for the use of their powers, and a generally pragmatic, logical world view. This is described by the Contractors themselves as a lack of emotion in general, but not much evidence exists to verify this claim. Many Contractors exhibit at least some forms of emotion, including love, joy, guilt, and sorrow.
Later in the series it is explicitly revealed that Contractors are fully capable of experiencing emotions, but that those feelings do not play a prominent role in their thought processes. They are described as being totally rational, and totally amoral. The general rules seems to be that they feel no guilt, and they cannot form or retain social bonds. A normal Contractor cares, in essence, only about their own well-being, and everything is reduced to a cost/benefit analysis: Any other issues like laws, emotions, or the question of the sanctity of human life are irrelevant.
How a person becomes a Contractor hasn't been revealed. Shion Pavlichenko was born as contractor, and Suou became one after she absorbed Hei's powers, and, in the Darker than Black: Shikkoku No Hana manga, a character named Harvest has the ability to make people Contractors, but all other Contractors, such as Tanya, have become what they are via unknown means. In Kuro no Keiyakusha Gaiden it is revealed that Contractors are defects that emerge when humans are unable to become Dolls.
A list of known Contractors: