Throughout the series, Hei is required to bear many names and identities due to the life he leads. It is unknown whether Hei was originally a codename or his real name, however it is used frequently throughout the series to refer to him personally. “Hei” translates to “Black” in Chinese.
In public view, he goes by the name Li Shengshun, masquerading as a foreign exchange student from China staying at the Umitsuki apartment block, Li is known for being polite and well mannered, while also carefully distant; he is also known for having a notoriously huge appetite. As a part of his covert missions for the underground organisation known as the Syndicate, Hei takes on various part-time jobs that allow him to track to his targets.
Infamously known as “the Black Reaper” in the underground, or by his Messier Code “BK-201” by the authorities, Hei is both a highly skilled assassin and infiltrator, alongside being a Contractor with the ability to emit electricity throughout his entire body. In combat, Hei requires either direct contact or a conductive medium (i.e. metal, water, etc.) to affect his opponents; he is also able to control the voltage emitted, allowing him to stun or kill a target as needed. During missions, he typically dons a distinctive white mask and dark bullet-proof coat, alongside equipment attached to his belt which allows him to project wires to either facilitate a kill or to grapple with his surroundings for a greater range of movement (i.e. swinging, abseiling, etc).
Prior to the story, Hei, already infamously known as “The Black Reaper” was not a Contractor, but gained his abilities following the disappearance of his sister, along with Heaven’s Gate, during the Heaven’s Gate Conflict. It is revealed that Pai’s power had became Hei’s own as she continued to live on within him; due to this, Hei’s inherited powers do not require a payment as his sister’s sacrifice, the cost of her own life, rendered the contract “complete” and annuls any future payments.
In the final story arc, Hei’s/Pai’s ability is revealed to do more than simply emit electricity, rather it enables them to manipulate electron particles that can disturb matter, even all the way down to the quantum level when enhanced by the Gate-related artefact known as the Meteor Shard.