So basically in short there are a brother and a sister with very harsh parents who don't really care that much for them(about like Harry and the dursleys on Harry Potter if u have seen that),but one day,they is walking around and see a black cat with one red eye and one blue eye.They both pet it and they both move on.they go home that night and find there parents simply gone.The cat however is in the house and it's red eye is glowing brightly.It notices them and moves toward them,but pauses,not sensing the harshness and hate kit had sender in their parents, so it decided not to harm them.It explains it is a grim reaper working for both heaven and the underworld(that's why it's eyes are that color).It says their parents where on the list for being in a orginazation that worked with demons,and needed to be dealt with by the angels,but dosnt explain more about that.Instead,the cat says he is looking for the leader who supposedly became half demon.It asks them if they would like to help him find this person and they both agree and it goes from there.