Prior to the events of Darker than Black, Hei was just a normal boy who lived with his sister Bai. During a meteor shower, his sister became a Contractor (BK-201). She ended up joining a secret organization called the "Syndicate". Although Bai was a Contractor and Hei despised the thought of taking another persons life, he joined with her in order to protect her. At some point, they met Amber. Even before he gained his powers, Hei was an expert fighter and killer, earning himself the name "The Black Reaper". During the events in Heaven's Gate (known as "Heaven's War"), Bai went missing, along with all of Hei's team. As a result, Hei inherited Bai's power to manipulate matter to the atomic level. From then on he vowed to find out what happened to his sister.
Hei first appears as he's hunting a Contractor for the Syndicate, which he effortlessly completes. Using the name "Li Shengshun", Hei rents an apartment where he begins his new life. He does various missions for the Syndicate, while working odd jobs to get closer to his targets. One in particular being the time when Hei infiltrated PANDORA working as a janitor in order to retrieve the "Meteor Fragment". At one point, Hei meets Misaki Kirihara, a police officer who he crosses paths with on almost every occasion. Some time later, Hei's reunited with Amber, a young woman he used to work with and appeared to have some sort of relationship with. Now the leader of her own organization "Evening Primrose", Amber reveals her plot to destroy Hell's Gate in order to ensure the existence of Contractors. Although Hei declares her his enemy, Amber still makes Hei her main focus, ensuring his well-being above all else. During the events that take place in Hell's Gate, Hei, Yin, and Mao confront Amber in order to change her mind since her plan would kill millions of non-Contractors. Instead of going through with her plan, Amber sacrifices herself for Hei. Soon after, Hei, Yin, and Mao disappear. It's revealed that they defected from the Syndicate and are now on the run. Posing as newlyweds, Hei and Yin attempt to travel undetected, but it fails as they continue to get hunted down by the Syndicates Contractors. At some point, Yin gets captured.
Hei reappears in Gemini of the Meteor, it's implied that he killed a fake version of Suou's father and later kills April. Mistaking Suou for her brother Shion, Hei captures her. Losing his powers in the process due to a machine that was developed to strip Contractors of their abilities. After finding out she's not Shion, Hei decides to go with her to Japan, where Shion really is. They get accompanied by Mao, who possesses the body of Suou's pet squirrel and July. While looking for Shion, Hei trains Suou, and searches for Yin at the same time. At some point, they find Yin. Unable to kill her and powerless without his abilities, Hei is unable to save Yin. The group later runs into Shion and Suou's father, finding out that Suou died years ago and this was just a fake one created by Shion. During the final events in Hell's Gate, Hei stops Suou, who's slowly losing her memories, from shooting Yin. When Misaki arrives, BK-201 (star) begins to shine bright over Hei and Yin as they vanish. It's apparent that Hei's still alive as his star's still shining in the sky and he's seen walking with Yin in his arms.
In the beginning, although he possessed human emotion, Hei attempted to act like a common Contractor. He cared deeply for his sister, going so far as to joining an organization and learning to become a killer just so he can be close to her. It was also appareant that he had some sort of relationship with Amber. Later on in the series, Hei developed a strong emotional attachment to Yin, putting her well-being above his own or anyone else. His personality changed greatly after losing Yin. He became an alcoholic, being more aggressive and ruthless than before. Even going to such lengths as hitting Suou whenever she did something wrong. He reverted back to his former self when he found where Yin was being held.