Hei (黒（ヘイ), Hei), also known as the Black Reaper (黒の死神, Kuro no Shinigami) or BK-201, is a Human who gained Contractor powers. He is the older brother of Bai and the main protagonist of the series.
Hei is a young man, with black hair and dark blue eyes. When off duty, Hei can be seen in the casual attire of a Chinese transfer student. When operating as a Contractor, however, Hei dons a black, bullet-proof trench coat and a white mask.
Later, after the events that transpire at Hell's Gate, Hei allows his hair to grow out to shoulder length; though he often keeps it tied in a ponytail. He also now wears a purple and black jacket and a snow cap on his head. Hei also appears to have neglected shaving.
Shortly after this, however, Hei cuts his hair back to normal length, shaves, and returns to his "Black Reaper" attire.
As a member of the Syndicate, Hei regularly takes on false identities and personas for his missions and day to day life, such as Li Shunsheng (李舜生 (リ・シェンシュン), Ri Shengshun): a good-natured, timid Chinese exchange student who tries to help others but otherwise generally keeps to himself.
Hei is a calculating assassin. Unusually for a Contractor, Hei exhibits and sometimes acts upon emotions. Hei also has a huge appetite, often surprising people with how much he can eat. Despite having the power of a Contractor, Hei still has his human emotions. He deeply cares for his sister, Bai, and searches for her after her disappearance.
After Yin's disappearance, there is a drastic change in Hei's personality. He takes up drinking, is more ruthless, quicker to anger and would often lash out at Suou, striking her across the face whenever she said something that angered him. He was rarely seen angry prior to this, but since learning of Yin's whereabouts, Hei reverts back to his previous attitude.
Before he became a Contractor, Hei was a caring brother who looked after his little sister Bai. Despite his age Hei seemed to be the guardian of Bai. During a meteor shower his sister became a Contractor, with the ability to control and change matter on a molecular level, causing her to become the original BK-201. Soon after, Bai was recruited into the Syndicate. Although Hei hated killing, he did so anyway so as to be with his sister, and not view her as just a senseless killer. He became so good at fighting, that even before he obtained his Contractor power, he became known as "The Black Reaper".
During the "Heaven's War", Bai went missing with the rest of Hei's team. Hei then came into possession of Bai's power and took over as the new "BK-201". He continued to serve the Syndicate in hopes of finding out what happened in South America's Heaven's Gate, and more importantly what happened to his sister. Hei also has a past history with the leader of the Evening Primrose and former Syndicate co-worker, Amber
Peak Athlete: Hei's contract powers are especially useful because of his amazing agility, speed, endurance, dexterity, and natural strength.
Expert Hand-to-Hand Combatant: Hei is obviously extremely skilled in martial arts and is an accomplished acrobat easily able to do backflips and mid-air cartwheels with little effort in terms of physical strain. Hei's natural abilities are so impressive that even without his contractor powers Hei is easily able to take on other contractors. A combination of these physical attributes coupled with what appears to be the most powerful contractor power as well as a lack of obeisance arguably makes Hei the strongest character in the series.
Molecular/Electricity Manipulation: Hei possesses the ability to generate and discharge electricity through conductive media (i.e. he cannot discharge it through air or concrete) at sufficiently high amounts to kill grown men or short out a building's electrical systems. The power is more than simple electric shocks, as he can alter matter on the quantum level. He has shown the ability to transform integral particles on the quantum level. He is capable of special particle luminance, meaning he is able to change molecules similar to how humans are changed into contractors. This ability is also one of the most, if not the most, powerful and coveted powers known as it is the only one capable of altering gate particles at will. This ability is not actually his as he is really only human, but it was granted to him when his sister fused with him.
- Obeisance: None. This inheritance means that Hei doesn't have an Obeisance unlike other contractors. The reason behind this is that he is actually a human who has fused with his sister a contractor, therefore he has no price to pay.
- Mask: Hei wears a white mask with a purple lightning symbol over the right eye.
- Knife: His trademark weapon is a knife on a metal wire, which he uses as an offensive weapon, a grappling hook, or as a conduit through which to discharge his electricity.
- Wire: In addition to using it with his knife, Hei also uses his wire with a carabiner attached to the end, which allows the wire to anchor to any object that it wraps around.
- Black coat: Hei's long black coat is bullet proof.