LogFAQs > #964575441

I'll give you an abridged version that trims away some of the details that might confuse you further.

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Ansem/Xemnas/Xehanort

Ten years before Kingdom Hearts 1, there are three Keyblade-wielders apprenticed to a Keyblade Master named Eraqus. Their names are Terra, Ventus, and Aqua. The story of the "Xehanort" you speak of begins when a Keyblade Master named Xehanort takes an interest in Terra. Master Xehanort seeks to forge a legendary weapon, the X-Blade, in order to gain mastery over a powerful force known as "Kingdom Hearts". To this end, he seeks to distribute fragments of his heart into thirteen vessels, as part of a plan to forge the X-Blade. Xehanort plans for Terra to be his third vessel; the former succeeds in taking over Terra's body. Later on, Xehanort in Terra's body is defeated; after his defeat, his memory is erased. The resulting being, with Terra's body and with neither Terra nor Xehanort's memories, becomes the entity you know as "Xehanort". This Xehanort performs dangerous experiments on his own heart, which ultimately result in his transformation into a Heartless, which you know as Ansem. As a byproduct of Xehanort's tranformation into a Heartless, Xemnas is also born. He is the Nobody of Xehanort; Nobodies are essentially the body and soul left-over when a person of strong will is transformed into a Heartless.

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Ventus/Venitas/Sora

Fourteen years before Kingdom Hearts 1, Xehanort performs a dangerous experiment on his apprentice Ventus. This experiment results in the birth of a new being, known as "Vanitas". He is the physical embodiment of the darkness in Ventus' heart. Vanitas and Ventus are the key components to Master Xehanort's original plan to forge the X-Blade. The X-Blade is forged, but it is broken soon after, thanks to Mickey, Aqua, and Ventus. As a result of the X-Blade breaking, Ventus' heart is shattered. It would have faded away completely, if it had not found refuge inside four-year-old Sora's heart.

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Sora/Roxas/Xion

When Sora stabbed himself at Hollow Bastion, he became a Heartless, which then resulted in the birth of Roxas, Sora's Nobody. Then after the events of Chain of Memories, Sora's heart and memories are scattered; they eventually find themselves into Roxas. As for where Xion comes in? The Organization created a replica known as "Xion" in order to replicate the power of the Keyblade. They designed her in such a way that she inherits the ability to use a Keyblade from Roxas, who had joined the Organization six days prior to Xion's birth. She then becomes connected to Sora through Roxas; the fragments of Sora's heart begin drifting into Xion, as well. Eventually, Xion absorbs too much of Roxas and Sora's heart/power to the point that the only way to restore Sora's heart is for Xion and Roxas to die. Xion forces Roxas to kill her, and then Riku confronts Roxas. Roxas is then forcefully assimilated into Sora's body at the end of the prologue of Kingdom Hearts II.