Hei

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.

Appearance
Hei is a young man, in his mid-twenties, with short black hair and black 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.

Personality
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 Shengshun (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 and is quick to anger. He was rarely seen angry prior to this, but since learning of Yin's whereabouts, Hei reverts back to his preivous attitude.

History
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, which might have been the one that brought the Meteor Fragment to Earth, 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 happen to his sister. Hei also has a past history with the leader of the Evening Primrose and former Syndicate co-worker, Amber.

Contract
Molecular Manipulation: Hei possesses the ability to generate and discharge electricity through conductive media, such as water or metal, at sufficiently high amounts to kill grown men or short out a building's electrical systems. The power is more than simple electric shocks, he can alter matter on the quantum level. He has shown using 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 power, however, 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 a 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.

Trivia

 * Hei's eye color is inconsistent through the various media. While most shots show that his eyes are black, numerous anime and manga images depict them as always being red, while others, show them to be blue.