Contractors weren't always contractors. They used to be regular human beings just like you and me, but after the mysterious appearance of Heaven and Hell's Gate which altered the appearance of the skies twelve years ago, people started acting differently. After the spontaneous creation of the gates, the natural sky disappeared along with the natural stars. Immediately after the stars changed, people began to come into possession of strange powers.
Each contractor had a power and a star unique to their own, meaning that each star had a contractor to come with it. Contractors were kept secret from the public but could only be so for so long seeing the rising number of mysterious deaths. The abundance of contractors however could only be determined by how many stars remained in the fake sky.
Since the identities of every contractor are impossible to comprehend, they have used each star's Messier Code to identify when they are actively using their abilities or when they have perished. Hei is an infamous contractor with the Messier Code of BK-201, however when he is not in action, he is Lee Shenshun, a Chinese exchange student attending university in Tokyo. The general public have given him the nickname "The Black Reaper." after his contractor's attire of a black, bulletproof coat, and mask. He had the ability to generate electricity until recently, when the government had developed a weapon that exorcised him of his powers.
Unable to fight with electricity, he was put to a disadvantage when fighting off all of the different factions that were out to get him. Yin is a doll, an emotionless spirit medium with the ability to use her observation spirit to track enemies and Hei's targets.Oh and also Hei's lover. An observation spirit is only able to 'observe' through a medium. Yin's medium is water, but she does not have to be in direct contact of water in order to use her observation spirit.
Contractors are supposedly emotionless, but unlike dolls they can act like regular human beings. Hei however, has been able to retain his emotions, and his one ambition was to find his sister, also a contractor who has disappeared on the day of the 'destruction' of Heaven's Gate in South America. Only to discover that his sister is gone, Hei had all but given up until Yin had begun showing feelings for him. This was unlike a regular doll. As a doll, Yin was supposed to be emotionless. But this was a sign that both of them were different than the rest of the contractors and dolls. They were evolved from the rest.
Eventually, after many years and plot turns and twists, Hei and Yin had managed to destroy Hell's Gate in one fell swoop, but have been lost in the process...or have they?
Author's Note
Meh. Well I couldn't explain the entire plot in one chapter because of laziness. If you want a better understanding of the plot, then watch the anime. Its worth it.