Light and dark balance out one another the same way the good and the evil balance out each other. They cannot be destroyed or created. Without light, darkness cannot exist and without darkness neither can light. They balance each other out.
The same could go for my Brat and me. Without her, I'd be a pessimistic nightmare and if she didn't have me with her all the time, she'd be too hyper. We balance each other out and if one leaves... the other is doomed.
I am the Brat's Dark side.