This manga is mostly about a girl that meets a boy that kills for a living. She has a strong sense of judgment so she doesn't want him to kill anyone anymore. As the story goes on several other killers that work in the same kind of "company" as him appear and they all become friends. She learns more about Ogami (the boy) and all about his situation that he's in. She tries to help him. In the more recent chapters, it is about how their friendship is tested when the other killers go out and try to kill Ogami.
Overall, its just about a girl helping a boy and lots of killing and friendship in between.
I really like this manga so I think you should defenitly read it once you get the chance! But I am going to warn you now, this manga can get complicated and seem like its making no sense at all but that's just how its written.
So I hope you read and like this manga!!
