High school boy Keita lost his mother when she encountered a copy of herself. His only close friend is a Akane a young woman who was a childhood friend. She made it her task to keep an eye on Keita in place of his mother. At some point she developed unspoken romantic feelings towards him.
Soon we encounter Kuro a young girl with a big appetite and childish manner. She tells us she is a guardian of the Doppleliner System. She keeps an eye on Tera, the life force people are born with. She is seeking revenge on her brother for killing her fellow guardians. It turns out that guardians and humans can make a pact and gain powers.
Despite Kuro's girlish appearance this series is full of bloody violence. I read volume one of the comic book called Black God created by Lim and Park and thought the fights were brutal. Then I saw the subtitled anime. Still brutal. I recommend you take the time to watch one episode per day rather than rush to get to the final conclusion. As often happens the comic and anime diverge. Perhaps the comic is still ongoing so a different storyline had to be made for TV. The anime definitely comes to a happy/sad conclusion and there is no way for it to continue.
There is a free, streaming dubbed version on the Internet and pretty soon Bandai will release the DVD.