Legend of korra

Bryan Konietzko imagines that Raava and Vaatu were born from the energy of the primordial Tree of Time. When envisioning the spirits' conjoined form, Konietzko also imagined that Raava and Vaatu would fit back-to-back as opposed to their initial spherical shape in "Beginnings, Part 1", with either spirit's "blank" side covering their counterpart's.

One of the most unknown questions: Why did Raava turn to Wan and Vaatu?

The moment Raava and Vaatu were separated, she started shrinking, as with the dark spirit on the loose, the light for which she stood began to fade. Angered, Raava turned to Wan, who believed her to be nothing more than a bully. She explained who she was and that by helping Vaatu, Wan had let chaos back into the world.