Kingdom Hearts
The Heart of All Worlds
The great heart at the center of the series' cosmology, pursued through both its true form and artificial imitations.
Kingdom Hearts is described as the source of light and the heart of all worlds. Ancient accounts tell of an age when the world was united in that light, before conflict divided it. The χ-blade is its counterpart and the key associated with access to its true power.
Not every heart-shaped Kingdom Hearts seen in the games is the same construction. In the first game, Ansem, Seeker of Darkness gathers the hearts of fallen worlds and creates a doorway to a Realm of Darkness. In Kingdom Hearts II, Xemnas creates an artificial Kingdom Hearts from people's hearts released by Keyblades. Those attempts should be distinguished from the true Kingdom Hearts Xehanort seeks through the χ-blade.
Birth by Sleep shows Xehanort attempting to create the χ-blade by merging a heart of pure light and a heart of pure darkness using Ventus and Vanitas. This however creates an incomplete χ-blade which ultimately fails. in DDD it is reavealed that in order to create the true blade, 13 darknesses must clash against 7 lights.
In Kingdom Hearts III, the conflict between seven guardians and thirteen seekers completes the weapon, allowing Xehanort to try to erase the existing world and begin again. After his surrender, Sora and his allies close Kingdom Hearts.
The recurring numbers are related to the story of the χ-blade's division into seven light fragments and thirteen dark fragments. This symbolism is seen throughout the series in the form of the seven Princesses of Heart, seven Guardians of Light, Organization 13, thirteen Xehanort vessels, and thirteen ancient Darknesses.