The rest is here:
Kairi’s Lament

This Story is non-canon): ——Kairi and Sora were friends since they were children. Along with their close friend Riku, they spent many wonderful years together in peace and happiness. As they grew older however their friendship grew into something more and they fell in love. Those were happy times for the both of them and they swore that they would love each other and be together always. Sadly, tragedy struck the young couple. Riku had gone off to war and had been reported missing in action …