In Tokyo, Hei works for an organization known as the Syndicate and earns the nickname "Black Reaper" (黒の死神, Kuro no Shinigami) due to his skills as an assassin.

Hei also appears in the sequel Darker than Black: Gemini of the Meteor as the mentor of young Contractor Suo Pavlichenko while searching for his partner, Yin.

Hei's character was created by anime director Tensai Okamura, who cited espionage, ninja, and dual-personality influences in contrasting his daily life with his gruesome work.

Yuji Iwahara faced multiple difficulties in making his design appealing, while Okamura was surprised by Hei's popularity in the series with male and female viewers.

Critical response was mainly positive, noting his dual personality; Hei was initially seen as mysterious at the beginning of the first series but quickly developed into more a caring character.

Okamura cited a number of influence for its creation, including the concept of a ninja-type spy working for Tokyo which he saw when he was young.

Okamura wanted Codename BK201 as the series' title, but the anime staff found it too weak and Darker Than Black was chosen by a member of MBS TV.

[3] As Hei's state of mind remains unknown in early episodes of the series, Okamura said that the lyrics of the anime's theme song,"Howling" by Abingdon Boys School, give multiple hints about how the character thinks.

Yuuichi Nomura, known for writing tragic storylines, wrote the episodes about Hei's issues with his former Amber who holds romantic feelings for him.

Main writer Shōtarō Suga wrote the episodes in which Hei befriends a spy who steals an item from the Pandora organization that oversees the supernatural Gates that created the power of Contractors.

Hei eventually betrays the Syndicate to protect humans and Contractors as a result after being motivated by the late people he has a vision of.

[5] During Gemini of the Meteor, Okamura considered Hei a "rough, working guy", and Suo Pavlichenko took over the lead role to make Darker than Black more innovative.

[8] Instead of a happy ending, however, the original video animations and the sequel Gemini of the Meteor strike a noir note based on Gee's comments.

[14] Because of Hei's cold and sometimes-blank appearance, the staff decided to draw his eyes without highlights still relatively unusual in anime.

[19] The fourth episode gives a glimpse of the character's weakness, however, and a tragic fate for Yin as Hei is pushed with the idea of killing her but he refuses.

Kiuchi said that Hei is a professional killer who carries out missions as a Contractor as ordered by the Syndicate and assumes the identity of Li Shun Sheng, a Chinese university student who lives in a nondescript apartment.

The actor called Darker than Black and Crayon Shin-chan his favorite voice-dubbing experiences, and was glad that Hei's voice was close to his own and he did not have to make eating noises.

[23][24] Anime News Network called Liebrecht's voicing of Hei "a schizophrenic variation" of Syaoran from Tsubasa: Reservoir Chronicle.

As a civilian he is Li Shenshun, a good-natured Chinese exchange student who lives in an old apartment complex for foreigners and gets by with temporary part-time jobs.

He is aided by Yin, an emotionless human known as a Doll, and Mao, a contractor trapped in a cat's body and overseen by the agent Huang.

[27] Hei began working with the Syndicate as a child assassin to protect his sister Bai in South America during the Heaven's Gate war, and his goal is to find her again.

Depressed by Yin's apparent death, Hei remains in a restaurant drinking alcohol his hair growing up in the past time.

[41] Hei and Yin also appear in the manga Shikkoku No Hana, set after the first anime, in which they investigate how a Black Dandelion is giving people Contractor power in accordance with Amber's last wish.

Harvest, a former ally of Amber, developed Contractor powers with a Black Dandelion provided by the organization Pandora while trying to prove his superiority over Hei.

[46][47] According to a Fandom Post retrospective, Hei and his voice actors were popular with male and female viewers of the series; the character was nicknamed the "Chinese electric Batman".

[57] Hei's character development in the series' final episodes was praised, as he interacts with Yin and Amber while ending his journey to learn the truth about his sister's fate.

Originally considered a cold-hearted Contractor, the revelation that Hei is a human with supernatural powers inherited from his sister makes him a "third option" in the narrative.

[61][unreliable source] Maxibe Gee of the University of York called the character a "posthuman noir" protagonist similar to Rick Deckard in Do Androids Dream of Electric Sheep?, Motoko Kusanagi in Ghost in the Shell and Vincent Freeman in Gattaca (1997) due to their shared backgrounds and cruelty.

[9] Hei's Pierrot mask is considered a defense mechanism compared with the darker personas of famous characters such as Himura Kenshin or Vash the Stampede.

[63] Japanese voice actress Nana Mizuki acknowledged Hei's popularity with the female audience due to his calm demeanor and the gentleness of his Li persona.