Refers to the concept of the protagonist living in a house with other characters typically of the opposite gender. They might even be involved in a romantic relationship, but there is no legal relationship between them of husband and wife.
I fell in love. Her name was Shirasaki Yukino. I could only look at her from a distance. However one day I heard that she already had a boyfriend. Even if I love her, I can’t have her. The devil sent me an invitation. I can make her mine!