Legend tells of seven princesses, all of purest heart, that rule over the worlds. They are also the keys to Kingdom Hearts. But, there is another legend, of another protector of Kingdom Hearts. A guardian soul. A soul so strong and clean it cannot be broken. This soul was created during the first great crisis, when someone first attempted to breach the door to light without permission. The soul was given to a child. This child was tested on wits and ability as they grew older, and was granted knowledge beyond all years, giving them the ability to remember and connect with themselves in the next life. so that they could pass knowledge on through the generations.
Centuries passed, and so did the soul. It was named the Soul of Serenity, because each person it embodied was given an inner peace, or calm, that they could tap into in times of need, and gave them great diplomatic skills and a clear mind, which were often used during world crisis'. But, somewhere along the line, one child became corrupt. They didn't want their power, and refused to accept their fate as Guardian, rebelling in any way they could, before eventually becoming something of a criminal, or demon. The Soul became shadowed, clouded with its new-found feelings and emotions that clashed and fought inside. It's power became too great, and finally, after the Guardian passed on, the current princesses of Heart split the Soul in two. They realized it had become too powerful, and decided this was the only way to keep the Soul safe. Finding more than one heart with the strength and purity required for the job would be too difficult, so instead of two separate people entirely, the Soul split the next person's heart, creating the first Nobody. This Nobody was unlike all* others who would come after them in time, because they carried half of their other's heart. They two people were kept as far apart as possible, so that they wouldn't merge and become too strong again. Years passed, and this new plan seemed to work. But, as the years went on, it became harder and harder to find purity and hope in children's hearts. For a while, the Guardian just... disappeared. They didn't need the Guardian anymore, and so the legend of the Guardian of Heart was forgotten.
Only a select few people remember the legend, and fewer still tell it. Until the day when it is needed again, the Guardian's Soul sleeps, awaiting its awakening.
Now, with an impending war that no one is prepared for on its way, the Soul will need to awaken, very soon.
*this is meant to mention that maybe I think/I'm guessing that that was the deal with Roxas and Sora, that he held part of his heart and stuff, but I'm not sure, so correct me if I'm wrong!