In this world, monsters are real and are actually something like pets. You can either be a human who hired a monster maid or a monster that was hired.

If you're a human: You wake up one morning and realise...your house/apartment is a mess, and you're too lazy to clean, so you've decided to hire a maid from the Monster Pet Service. After a few days you hear a knock on the door, it's here

If you're a monster: You are whatever monster you want, a few days ago a new request was received. Now, after a few days of determining and decided by your higher ups, you've been sent out to clean someone house/apartment. Goodluck