KH: BBS (Major spoilers ahead)

[Teroniss]

So I more or less just finished a semi-translated version of Kingdom Hearts: Birth by Sleep, and jesus christ this story is getting convoluted. One of the main character's, Ventus, is apparently a Heart born from someone's dreams (Wasnt to clear on that yet, need the English version to be 100%). By the end of the game, he loses his body and become's in danger of becoming a Heartless, and merges his body with a young Sora to prevent it (Why Sora has a Keyblade). Aqua, another main character, uses her Master's Keyblade to create Castle Oblivion to restore Ventus eventually. Sora in KH1 loses the Ventus half of his heart, which is why, as a Heartless he is able to maintain control and why his body was capable of being restored. The Nobody born out of that, Roxas, is actually born from Ventus's heart, which is why he looked like Ventus and not Sora.

Also, apparently Vanitas, like Ventus, was born from dreams, but takes the appearance he takes without his helmet because of Ventus merging with Sora's heart.

God my head hurts........damn you Nomura!

[Jairus]

I think it's more like Ventus is a complete being who had all of the darkness removed from his heart - thus creating Vanitas - which almost killed him, but by chance he managed to meld with Sora to a limited degree which allowed Ventus to survive. Ventus later loses his heart, but because of his bond to Sora his heart is able to survive within Sora (which as you mentioned lets Sora wield the keyblade and is also why Roxas looks like Ventus, oh and also why Vanitas looks like an older Sora). Aqua made Castle Oblivion to protect Ventus until his heart and body could be reunited. The idea I heard for Sora able to stay in control while being a Heartless is because A) he was so strong of heart and B) he had so little darkness (which might be explained by Ventus being there too), while Roxas greatly resembles Ventus and may have even had Ventus' heart explaining why he seemed to have emotions.

And yeah, it's getting complicated. Let's not even get into how Xion is somewhere inside Roxas who is somewhere inside Sora, and Sora is somehow going to be able to bring all three of them back despite Roxas and Ventus being so similar. Damn you, Tetsuya Nomura.