In a random AU, there are not only monsters and humans, but some have small companions called Bitties and Borrowers. Bitties are hand sized monster companions that help and keep you company. Borrowers are like Bitties but human version. And in this universe, they're not only for company. For you see the humans in this AU, can wield magic. Mages, wizards, and witches of all sorts live here in this world. And these small companions provide an important role to play in the magic user's lives. They're used in battle against other bitties and borrower's and they have magic themselves that can enhance their "owner's" powers. 

This way of living has been used for centuries. But what the humans DON'T KNOW, is that Bitties and Borrower's can do more magic and have more abilities than they let on. They have their own society out in the wild and even though some are sold in shops, ALL of them know a sacred rule when it comes to finding their mates. NEVER LET YOUR OWNER KNOW THAT YOU CAN GROW AND SHRINK IN SIZE. ONLY FOR YOUR MATE TO KNOW. 

But what happens if more and more of these "owner's", monster or not, are your destined mate? Well then. That's a completely different ball game, isn't it?