"I know now, without a doubt. Kingdom Hearts... is light!"
At the beginning of time, there was nothing but darkness... Then, the first heart came to be. That heart shone so brightly that it almost completely wiped out the darkness. The first heart shone completely alone, however. The First Heart, wanting to not be alone anymore, split itself into millions of millions of smaller hearts, each heart becoming a world of its own and begatting other hearts. The remnants of the First Heart became known as Kingdom Hearts. Kingdom Hearts would shine down on the many worlds and people would bask in its glory. It gave passageways to travel between the worlds and those who travelled along it were given keys to open the doors along the way. But with it's diminished size, Kingdom Hearts could not shine everywhere at once, as the First Heart had, and so, the darkness came back. As the darkness crept into the hearts of people, they became jealous of Kingdom Hearts' power and sought it for themselves.
A war broke out between those with Keys, those who wanted to serve Kingdom Hearts as loyal protectors and those who wished to take the power of Kingdom Hearts for themselves. It is said that, in the end, Kingdom Hearts destroyed all sides, deeming itself a prize that would not be won by anyone. Then, Kingdom Hearts sealed the doors between the worlds, knowing that the worlds were now better off without knowledge of it.
However, Kingdom Hearts was not able to change the fact that, as soon as you found the roads, you were granted a key. Hearts flock together and seek to find meaning in meaninglessness. So, when people fell through a crack in their world and onto one of the roads, their heart would latch onto the first they would find and create a Key for the wielder. Naturally, there are very few with such an honour. However, with Kingdom Hearts not being able to shine everywhere at once, the Heartless began to make themselves known. An unstoppable force of destruction that plunge worlds into darkness and consumes it, now that the First Heart cannot stop them everywhere. Kingdom Hearts decided to bestow upon those who wielded the Keys, the power to destroy the heartless and release the hearts that they have consumed. Key-Bearers, as they are called, now find their duty is to travel to the many different worlds and destroy the Heartless wherever they can find it.
Every Key-Bearer knows of Xehanort the Wise, the oldest living Key-Bearer. Some say that he was the sole survivor of the War, others just say that he is the strongest and wisest man to ever pick up a Key. What is known is that most every master of keys was trained by him at some point.
List of Jobs for the Job system.