Codename "Hei", the main protagonist, is an elite Contractor, with the Messier Code BK-201. Hei has a double persona in the series. In the general public he is referred to as Li Shengshun, a good-natured Chinese exchange student living in the Umitsuki apartments.
Operating as an assassin and stealth operative for the syndicate, Hei is involved with many incidents involving contractors. He himself is a contractor, who pays no price for his powers.
He is known as Kuro no Shinigami (The Black Reaper) to other contractors. This is because of his appearance during his missions, where he wears a distinctive mask and a black coat. Hei earned his reputation for being ruthless even before becoming a contractor.
Hei also appears to have a wide array of advanced equipment with him, including a bulletproof jacket, and wires that he uses to conduct an electrical current, which is also his contractor power.
Hei acts like a contractor in that he is emotionless and cold, he is able to put up a convincing facade of being a clumsy but otherwise normal person. Hei has managed to gain the affection of nearly every woman he has met but has never properly reciprocated that affection,
Hei, also known by his 'star name' BK-201 is a contractor and possesses supernatural talents. His ability is to quantum manipulation of matter, usually this take the form of electricity. Due to his unusual past his powers require no price.
Even before he gained his powers he was very skilled with knives and wires. After he gained his powers he began using the wires to conduct electricity. He has also been shown to use other weapons such as crossbows. His Jacket is bulletproof but this has no effect on his near-superhuman speed. He is also a master of stealth and can disappear in the time it takes someone to put on their glasses.
In his youth Hei cared for his sister, Pai. One night, during a meteor shower, Pai became a contractor with quantum manipulation abilities. Pai was the original BK-201.
Pai was recruited by the syndicate with Hei following not long after. Even though Hei disliked killing people, he wanted to be with his sister and not view her as a ruthless contractor.
They both fought in Heaven's War with Hei gaining fame as the Black Reaper. Something happened that resulted in Hei gaining Pai's powers.
After Pai's team had disappeared, and with no clues as to where she went, Hei stayed on with the syndicate in the hopes that he could find out information as to her whereabouts.
It is revealed later that Pai was actually fighting against the Syndicate and that it was slowly affecting her when Hei was carrying out their orders.
Throughout the events of Gaiden, Hei appears to be growing more attached to Yin, although it is never openly said so. Hei hugs Yin more than once, stating that she can believe in him, and even telling Yin to not leave him alone; a sentiment echoed by Yin at the end of the first series.
At the beginning of Gemini of the Meteor Hei begins drinking, possibly through the depression of losing Yin to Section 3. He's also sporting longer hair and stubble. Hei's personality changes dramatically from the first series. He is a lot more violent and quick to anger. He also makes no attempts to act like Li Shenshung anymore, keeping a cold demeanor most of the time.
He begins to revert back to his old self after he finds out where Yin is, this includes giving up on alcohol and regaining his tremendous appetite. At one point in Gemini of the Meteor, Hei comments that he will "kill her" when asked about Yin. In reality, Hei actually meant that he left her to die.
Hei loses his contractor powers throughout Gemini of the Meteor, but it appears he has regained them as of the end of the final episode as the BK-201 star is shining brightly once again.