The girl's name is Zhi Xin, a mute girl who hides her identity as a killer but has never killed anyone. She always has a bright smile on her face, but she is always crying inside for unknown reasons.
The boy (whose name has not yet been decided) and the girl have been together since childhood. He is also a killer. He regards the girl as another self in the world. He is responsible for everything the girl does not want to do. Anyone who makes a girl sad will never see the light of day. Their modus operandi is extremely high and there are no flaws. Because he didn't like the girl to come into contact with anyone other than himself, he eventually cut off the girl's hands and feet and imprisoned him in a place where no one could find him, except for himself and the girl.