If I recall, rumors were circulating about the origins of Xehanort, the lead antagonist of the series so far. These rumors included that Terra, one of the three playable characters from Birth By Sleep, becomes the Xehanort we know, the one who caused all the wrongdoings and misdeeds, including the formation of the dastardly Organization XIII. Well, fans, the rumors are seemingly true after all. Towards the ending of Terra's story, Master Xehanort, the Keyblade master whom Terra, Aqua, and Ventus sought to find, releases his heart and takes control of Terra's body, thus becoming the new Xehanort.
There's more. Vanitas, Master Xehanort's apprentice, also reveals his face towards the end of Ventus' story, and... wow! His face looks identical to Sora, the hero we all know from the series! Quite a surprise, huh? But, why? Why does Vanitas look like that? I'll tell you why. Vanitas, like Xion from Kingdom Hearts 358/2 Days, isn't a real being, but rather a physical manifestation. Vanitas was born from the darkness in Ventus' heart and from his birth came the threat of the enemies known as Unversed. Vanitas bears the face of Sora as a result of a side effect from Ventus' heart connecting with Sora's years before the hero first wielded the Keyblade. Vanitas was eventually defeated and wiped from existence by Aqua, forced to fight her friend Ventus when he was possessed by Vanitas. And with Vanitas no more, the Unversed were also destroyed for good.
Spoilers end here. Thank you, come again.