Hei

"A night sky full of cries, hearts filled with lies. The contract, is it worth the price? A soul pledged to the darkness. Now I've lost it, I know I can kill. The truth exists beyond the gate."

- Darker than Black opening

Hei (黒（ヘイ), Hei), also known as the Black Reaper (黒の死神, Kuro no Shinigami) or BK-201 (or "Chinese Electric Batman" by fans), is a contractor, or human who gained superhuman powers from a mysterious phenomen known as "Hell's Gate", in Hei's case, the ability to manipulate the molecular structure of objects and generate electricity. He is the main protagonist of the anime Darker than Black.

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 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 a secretive organization known as 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 happened to his sister. Hei also has a past history with the leader of the Evening Primrose and former Syndicate co-worker, Amber.

Hei is a young man, with black hair and dark blue 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.

Hei is a skiled in unarmed combat and knife fighting, and is almost superhumanly agile. The most powerful ability he possesses, however, is the ability to generate and discharge electricity through conductive media (i.e. he cannot discharge it through air or concrete) at sufficiently high amounts to kill grown men or short out a building's electrical systems. To make up for his inability to discharge through the air, Hei uses a rope dart made made from a metal wire to attack from a distance by piercing his enemies and then electrocuting them. The wire also has a carabiner attached that allows Hei to use it as a grappling hook.

The power is more than simple electric shocks, as he can alter matter on the quantum level. He has shown 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 as he is really only human, but it was granted to him when his sister fused with him.