Everything began the same. Sora was born on Destiny Islands, he was Riku's best friend and all was well. But the path diverged from the one that was considered destiny.
One day, a Heartless decided to conduct a special experiment, one that required a malleable mind and a heart full of light. He was drawn to the Islands of his past, and there he found a small boy. While the boy was out playing, the Heartless watched and waited in the boy's home. He slaughtered the parents brutally, and when the boy came home, stole him away. The boy's best friend stumbled upon the bloodshed the next day.
So began Sora's long stay in the dark.
"Ansem" carried him off to Radiant Garden, a world barely holding together. It was there he was told that his parents had abandoned him, and that he had been rescued. At first the boy would deny it, but in the end, he came to believe the man.
While he was there, he struck up a friendship with a girl. She soon vanished, though, and he was alone with nothing but shadows and the man he came to call Father. He was obedient, eager to please--a perfect candidate for the experiment.
Time went by. Sora came to appreciate the darkness, and his light slowly began to fade. His only friends were shadows, but he didn't mind. He willingly worked toward Ansem's goals, and eagerly led the charge of numerous attacks the snuffed out the light of entire worlds.
But one world shook him.
He returned to the Islands of his childhood, where he secretly prayed he could reconnect with his childhood friend: Riku. But when they met up, something was wrong. Riku couldn't remember anything about the boy. And so, heart torn in pain at what he considered a betrayal, he tore the world apart through the Heartless and swore that he would destroy Riku, too, if he ever met up with him again.