They were tryin' to find the 'Princesses of Heart'. Seven maidens with hearts of pure light, that when brought together could form a Keyblade of Heart that could unlock the way to Kingdom Hearts as well. Kairi was one of them- you may have met some of the others during your journey too. This Keyblade could unlock peoples hearts and fill them with darkness, but also free what was inside them, even if ya weren't a Keyblade Master... so when Sora finally found Kairis body in the darkness-infested remnants of Radiant Garden, and realized that her heart was inside him... he used it on himself, freeing not just his own heart, but hers as well. Her heart returned to her body, whereas Sora... well. You may have met Roxas around, have ya? He's the Nobody that was created at that time.
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You see, the reason Sora set out on his journey was because his world was swallowed by the Heartless. Durin' this attack, Kairi's heart was separated from her body, creating Naminé, and somehow her heart ended up inside Soras. He traveled out to look for Kairi and Riku with Donald and Goofy in a Gummi Ship, sealing the worlds and defeating those who'd try to use the Heartless for their own ends under Maleficent.
They were tryin' to find the 'Princesses of Heart'. Seven maidens with hearts of pure light, that when brought together could form a Keyblade of Heart that could unlock the way to Kingdom Hearts as well. Kairi was one of them- you may have met some of the others during your journey too. This Keyblade could unlock peoples hearts and fill them with darkness, but also free what was inside them, even if ya weren't a Keyblade Master... so when Sora finally found Kairis body in the darkness-infested remnants of Radiant Garden, and realized that her heart was inside him... he used it on himself, freeing not just his own heart, but hers as well. Her heart returned to her body, whereas Sora... well. You may have met Roxas around, have ya? He's the Nobody that was created at that time.
But if we go back to Castle Oblivion- some of the Organization were usin' Naminé to manipulate Sora by turnin' him into their servant. But at the same time the loyal members of the Organization were tryin' to work against them by manipulatin' Riku, who had entered the castle at the same time but from the bottom floor, in the Realm of Darkness. They tried to make his darkness overwhelm him, but together with me, he managed to not just undo their plans... he also defeated and overcame the remnants of Xehanorts Heartless that were still possessin' him. All with the help of my old friend... Ansem the Wise. Who had survived the fall of Radiant Garden into darkness, and now called himself DiZ.