First of all, I'll introduce myself since it's my first post on this forum. I've been a fan of the Kingdom Hearts series ever since 2005 and over the course of the years I've been putting together a bunch of theories about the series. The following is a copypasta from a topic I created under the same title over at the GameFAQs forum.
Feel free to post any insight or opinions you may have about this. It would encourage me to formulate more theories in the future!
Maugomale at GameFAQs said:By this time I'm certain all of those who spoiled themselves of the game's story want to know why the bloody hell does Vanitas look like KH2 Sora when unmasked. I've come up with a theory based on what I could understand from the story and I thought it would be interesting to share it with everyone:
Well now, for those who don't know, Vanitas is Ventus' darkness taken form, which was extracted from him by Xehanort.
Xehanort now had a powerful apprentice, so he made sure to dispose of what was useless for him. By ditching Ventus at the Destiny Islands, he allowed Ventus to connect his heart to Sora's in order to fill the gaps left by Vanitas' absence.
Let's suppose that even though Ventus and Vanitas were separate entities by this point, they still had some kind of tie, bond, that kept them linked. By connecting to Sora's heart, this caused a transformation in Vanitas, the result being the Dark Sora that you see in Ventus' Scenario.
I'm not 100% sure about this, since there are many things to be translated and considered before coming to a certain conclusion, but still I believe this makes sense to a certain extent.
Comment on this and tell me what you think! ;D
Feel free to post any insight or opinions you may have about this. It would encourage me to formulate more theories in the future!