What is Kingdom Hearts?

What is Kingdom Hearts, this is not just the title, but like a character of the game. Kingdom Hearts has appeared so far as a very large door (in Kingdom Hearts), and in Kingdom Hearts 2, Kingdom Hearts appears as a heart-shaped moon. Kingdom Hearts appears when a certain amount of hearts are gathered through the use of the Keyblade. The question of what powers can Kingdom Hearts perform for some are still unknown. I hope to discover what Kingdom Hearts could possibly do in future Kingdom Hearts games.