Goodnight and Billy are not the sort of couple that humans would think of as having a healthy relationship. For starters, one is a demon and the other an angel; that they are the youngest son of Lucifer and an angel of death doesn’t help matters in the least.
However, they adore one another in their own way. They always have one another’s back in a fight, which honestly for these two is the most important aspect of a loving relationship between a pair of monsters. That they’ll happily kill for one another is the icing on the cake.