Secret boss (spoilers possible)

[VenAqua]

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ive beaten the game with ventus and wondering where do i find the vanitas sentiment?
i know its kbg but i cant find it.

 

[Sacred X]

You need to beat the game with ALL 3 characters, then complete the final episode. Afterward, go to one of your 3 main data and he'll be in the Keyblade Graveyard: The Badlands.

 

[╬ProperArtist╬]

You have to beat all 3 main stories then the last episode and after you beat the final boss of the last episode, reload your game and he'll be available at the keyblade graveyard (second save option). He can be fought in all 4 scenarios.
Only after defeating him once on any scenario will MF unlock for the 3 main stories.
Since LOD is destroyed in the Last Episode he's not available to battle in that scenario