Hei, also known as the 'Black Reaper', is the main protagonist of the anime series Darker than Black. Hei is a Contractor working for the mysterious Syndicate, known for his deadly combat skills and ability to generate electricity. Despite the cold reputation of Contractors, Hei shows a surprisingly compassionate side, especially towards his allies. His quest to find his missing sister drives much of the story, making him a complex character who struggles with his dual identity as both an assassin and a caring individual.