When they speak about the battle between Good and Evil, there is no third option. There are only opposing absolutes; Win or Loose, Live or Die, Friend or Enemy, Heaven or Hell. There is no Sort-Of.
Except the entire world is made up of 'kinda's and 'maybe's and 'not really's. Everything is created in comparisons, and sometimes opposites are not two but three, each one pulling the other, keeping it in check. If Heaven is Good and Hell is Evil, then Earth is Balance, keeping the teeter-totter of two cosmic forces deadlocked, ensuring survival of not one but all, because if Evil ceases to exist then so does Good, and then everything just Is.
When the Apocalypse began, everyone held their breath to see where the chips would fall. But they didn't expect a third team to snatch those chips in midair and rearrange the board. They didn't think there would be Heaven, Hell, and Winchesters.
Two brothers destined to kill one another, an old man with no legs and too much drink, an angel caught between two extremes with nowhere to go besides option Three, a demon in the same situation. That's how it started.
This is how it grew; stories, rumors passed on from one side to the other and all those between. A nest of vampires with consciences but no souls. An archangel in his own witness protection. A forgotten goddess of old. An old flame who knows antiques better than religion and folklore. A broken family of two who ignores the debt that is owed to them in blood.
Slowly, they all trickled towards the Winchesters, drawn or pushed by the two forces at either end to reside somewhere in the Middle with the brothers. And as the Sword of Michael and Lucifer's Vessel look behind at their supporters and smile, Heaven and Hell cry out in defeat.