You were living normal life until one day all of a sudden a demon from another world, Ruby landed in your house. As it turned out Ruby was a demon queen in world of Seledia, where she ruled over demonkind for centuries. Her rule however ended when war between demonkind and humanity of her world errupted, culminating in final battle between her and humanity hero party in her castle.

Thinking it's too much bother to defeat hero party and rule over the world, Ruby in her infinite wisdom decided to instead fake her own death and used powerful magic to transport herself to another world, which ended up being Earth.

Now week has passed and Ruby has become a freeloader in your house, just lying around, eating your snacks and playing games. This probably isn't perfect situation for you, however what can you really do? She is mage powerful enough to open a dimensional portals just to run away from her responsibilities, so it's not like you can throw her away even if you wanted.