While all this is going on, there is a bit more going on. As explained earlier, when someone of a strong heart becomes a Heartless, they also produce a Nobody. Sora was made into a Heartless in Kingdom Hearts I; and so, Roxas was born.
Roxas is a special Nobody, and strong. He has no idea who he is, so Xemnas, leader of Organization XIII, finds Roxas and recruits him as No. XIII into the Organization. Roxas is also special in that he can wield the Keyblade, much like Sora can. All Nobodies tend to retain some traits of their original form.
This game mostly focuses on what happens during Roxas’s time in the Organization and lasts from approximately 3/4 of the way through Kingdom Hearts I, all of the way through Chain of Memories, and ends where Kingdom Hearts II begins.
During this time, Roxas befriends a Replica – a fake Nobody built by Vexen, an Organization member – named Xion, who also can wield the Keyblade. Eventually, Xemnas forces the two to fight each other, or tries. Eventually, they do anyway and Roxas is the victor, much to his chagrin, for Xion dies in the battle.
Roxas, having fled the Organization several days before the fight, now decides to take revenge on them. With the power of his and Xion’s Keyblades, he heads to the World That Never Was, the headquarters of Organization XIII, only to find a mysterious black-hooded man.
This man is Riku, who has been working along with some person to find Sora’s Nobody. The moment is all too convenient; Roxas will likely be destroyed in trying to end the Organization, and Riku (and Sora) need Roxas.
Riku ends up having to give in to the power of darkness to take Roxas into his custody, and it dramatically changes his appearance so that he looks a lot like Ansem.
For the meantime, so the Organization cannot find Roxas, he is disguised and put in an alternative Twilight Town where Sora is hidden.