Backstory.

Katsuki is a fiend created by the former angel Asbeel. Like many other Asbeel experiments Kastuki was originally a human soul that was transferred into something non-human. In this case, Katsuki's soul was placed into road kill. During the great purge, Katsuki’s life was about to be erased alongside other experiments like Megumi and Momo. It was not until Lord Grim came and saved their lives. Since then, Katsuki, Megumi, and Momo have worked under Lord Grim to take care of his daughter Yuno. When Yuno was sent to the mortal realm, Kastuki was ordered by Lord Grim to tag along to watch over her. He’s very good at tracking miasma and will also come with her on the investigation. Because of their circumstances over the years, Yuno and Katsuki have become very close. Their relationship is almost like an Uncle and his niece. Even though he tends to nag her a lot and always has to get her out of bad situations, Katsuki still cares deeply about Yuno and will do anything to help her albeit with reluctance. Katsuki is Yuno’s voice of reason.

When it comes to Takashi, Katsuki's mood starts to sour. Katsuki views Takashi as a distraction in Yuno’s life and doesn’t like his influence on her. From day one he always got a bad feeling about him and tends to not like being around him. That he thinks Takashi is a freeloader that tends  to mess up the apartments and has an unhealthy sleep schedule.