A long time ago, during the ancient war, Reshiram and Zekrom were once a single entity. They were always meant to separate after fighting a great battle and achieving true balance, but unfortunately before their victory was achieved their joined soul was permanently damaged. Moving forward, they can only take their true form with the help of a Sacred Taoist.

Ironically, Reshiram and Zekrom are famously a chaotic duo despite the balance they are meant to represent. One would think that their strange behavioral patterns are unfitting of deities such as they, but in actuality, the result of the clash of their personalities is what truly begets balance.

Being the Taoist of Balance is actually a very arduous task. The role requires managing both the hearts of Reshiram and Zekrom, keeping their powers in check as well as tending to their respective shrines, believers and disciples. Usually poisonkin are the ones who are chosen, but no one knows why other than the Great Mother.

...

And maybe the twin deities themselves.