Ten years ago, an inscrutable and abnormal territory known as Hell's Gate appeared in Tokyo, altering the sky and wreaking havoc on the landscape. The heavenly bodies disappeared, replaced by false stars. During this time, people possessing various special abilities emerged, each capable of different supernatural feats - these are known as Contractors. Each fake star has a corresponding Contractor, which reacts to their activities. As such, Contractors are usually identified by their star's Messier catalogue number. Kept secret from the masses, these individuals are known to murder in cold blood, smothering unnecessary emotions with logic and rationale. Their incredible abilities, however, are gained at the cost of their humanity - Contractors are thus named because of an involuntary compulsion to "pay the price" each time their power is used, which can range from eating particular foods and completing meaningless tasks, to self-harm and having their bodies change in peculiar ways. Various nations and organizations around the world train and utilize Contractors as spies and assassins, resulting in violent battles for valuable objects and information.

Hei Hei (黒, Black?) Voiced by: Hidenobu Kiuchi (Japanese), Jason Liebrecht (English)
The main protagonist and an elite Contractor with the Messier Code BK-201. Hei plays a gentle-hearted student from China named Li Shun Sheng (李舜生（リ・シュンシェン）, Ri ShunShen?), but when "working" he takes on the role of a seemingly (at first) apathetic and guiltless killer known also as The Black Reaper. Part of an organization of unknown ambitions known only as the Syndicate, Hei is searching for his sister Bai, whom he lost several years ago when the South American Heaven's Gate imploded. Hei's contract allows him to generate electricity, and he is hotly pursued by the Japanese government for his involvement in multiple assassinations. He acts as the spy/assassin of his team, swiftly dealing death without fail. His huge appetite and seeming lack of obeisance lend him more of a sense of mystery, even with his own team. He doesn't appear to have a remunition or regard for relationships outside of his team however. The only sense of concern and care seems to be revealed later on in the series, where Yin seems to be his main concern.

Yin (銀 In?, lit. Silver) Voiced by: Misato Fukuen (Japanese), Brina Palencia (English)
A supporter in Hei's team. Yin is a spirit medium known as a "Doll". Very passive and speaking rarely, she is described as having no emotion and incapable of such processes (however as the series progresses she shows a slight human side with hints of emotion). Her primary function is to locate individuals, usually assassination targets, for the team as she has the ability to see anything near water when she is touching some. Yin possesses memory of Kirsi (last name unknown), who lived in Finland learning to play the piano under the tutelage of famed pianist Eelis Kostinen.
In the manga, she resembles Kanon since she is also a "doll" though she has some of Kirsi's memories, making the method for creation of dolls into a point of speculation.

Contractor (契約者 Keiyakusha?)
Contractors are individuals who have gained superhuman powers due to the appearance of the Hell's Gate and Heaven's Gate. 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 remuneration, which is a bizarre, obsessive compulsive "payment" for the use of their powers, and a generally pragmatic, Machiavellian 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 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 to them.

Doll (ドール Dōru?)
Emotionless "mediums" created to mimic the forms of humans. Dolls can form a variety of functions; the most commonly seen in the series is their use of observer spirits, which serve as reconnaissance. Dolls have different mediums through which they can send their observer spirits and must be in contact with this medium while observing; for example, Yin's medium is water, through which she can send her observer spirit anywhere else there is water. A Doll can also act as the perfect sleeper agent. It can be given the memories and personality of another person and assume a person's personality perfectly, mimicking all their habits, dreams, and feelings until the time is right. Interestingly enough, a Doll by the name of Shinoda Chiaki seemed to have some "life" of her own by saving Hei from being killed by another Contractor. This suggests that Dolls, like Chiaki, can go beyond their "set" characteristics and become something else. Other examples include when Huang discovers Yin crying, July expressing confidence in himself and November 11 as a team, and Sakurai, a doll stolen by the yakuza, smiling as she runs away with one of their own.