A slave is a person who is the property of another and is forced to obey them. Throughout the history of mankind slaves have always existed to fulfill humans' every whim. Your slave can be a representation of slaves that have existed, exist or entirely from a fantasy realm.
But...that one time the machines made us all into docile slaves and turned us into human batteries with the Matrix program...the Matrix is a documentary
Poor Sunako, having to cook, clean, and do all the other house chores despite living with four able-bodied (hot) guys. She's basically working like a slave SO SHE FITS THE THEME OKAY!!