Vanitas

Vanitas (ヴァニタス) is a character and one of the primary antagonists in Kingdom Hearts Birth by Sleep. He is the sole apprentice of Master Xehanort, the game's main antagonist. Born from the darkness of Ventus's heart via Master Xehanort to further his evil plan, Vanitas's birth sparked the threat of the Unversed. It is because of his connections to Ventus, and Ventus's connections to Sora that he appears as a black-haired version of the Keyblade master.

Kingdom Hearts Birth by Sleep
After Master Xehanort viewed a battle between Terra and Aqua as part of their Master Qualification Exam, he left the room to find a hidden Vanitas awaiting him. The two conversed about Ventus, Vanitas claiming he still needed to make him stronger. Vanitas later appeared before Ventus in his room, speaking of Terra no longer being Terra, and vanishing into a corridor of darkness.

After departing from the Land of Departure and later reuniting at Radiant Garden to tackle a massive Unversed, Terra, Ventus, and Aqua went their separate ways. While Aqua stayed behind to contemplate, Vanitas appeared and approached her, the two fighting a vicious battle. After the duel, Aqua attempted to remove Vanitas's helmet, the dark enigma laughing instead, thus preventing her from doing so. Ventus overheard Vanitas's last few words to Aqua, returning to her side just as he left in another corridior of darkness.

Vanitas later found Aqua in Neverland, as well, Ven's Wooden Keyblade in his hands. The dark enigma snapped it in two, battling Aqua once again. He left Aqua exhausted and enraged at the thought of Ventus being in danger, just as soon as he had appeared, Vanitas vanishing yet again in a dark corridor.

When Ventus later returned to the Land of Departure, he was attacked by Master Eraqus. Terra came to his rescue and battled his old Master, throwing his friend into a portal he summoned much to Ventus's displeasure. The portal led him to Destiny Islands, where Ventus encountered Vanitas. Vanitas explained his birth from the darkness in Ventus's heart, and how he took his place as Master Xehanort's apprentice. This caused Ventus great pain, Vanitas vanishing as Ventus fled the world.

Eventually, Ventus found his way to the Keyblade Graveyard. Vanitas approached him, again speaking of Terra losing his identity and challenging him to a duel. Vanitas gained the upper hand mid-way, knocking Ventus down. King Mickey later appeared, healing Ventus. The two then proceeded to fight the dark enigma, Vanitas leaving yet again upon his defeat.

When Terra, Ventus, and Aqua approached their final battle with Vanitas and Master Xehanort at the Keyblade Graveyard, intense battles ensued. The two darklings met up with the Keyblade wielders, Terra charging them. Master Xehanort intervened, however, blocking Terra's path with massive rock formations. As Terra continued to pursue Master Xehanort, Vanitas went after Aqua and Ventus. The two fought valiantly against the dark warrior, but not before he returned to Master Xehanort's side after Ventus was frozen. He battled Terra alongside Master Xehanort, Xehanort later commanding Vanitas to go after Ventus and Aqua again.

Vanitas attacked Aqua from above, Ventus's warning too late. Aqua was knocked unconscious, Vanitas preparing the final blow while Ventus lay helpless and frozen. Ventus summoned all of his willpower, however, standing and facing Vanitas. The dark enigma revealed his face, revealing himself to be created from the darkness in Ventus's heart while summoning many Unversed to hold Ventus down as he forcefully intergrated into him. In a burst of light, Vanitas and Ventus were transported to the Awakening, Ventus's pillar now hosting a picture of Vanitas, as well. Vanitas wielded an incomplete version of the X-Blade.

As Ventus battled Vanitas in the Awakening, Aqua and King Mickey were forced to battle Ventus in the physical world as Vanitas possessed him, altering his form to have his possessor's golden eyes and suit. Ventus summoned the X-Blade, attacking. As Aqua defeated the possessed Ventus and destroyed the X-Blade, Vantias's metaphysical battle with Ventus came to an end after they shattered the pillar, with Vanitas fading out of existence as the unconscious Ventus was taken to the Mysterious Tower, Vanitas no more.

Appearance
Vanitas wears a black and red organic-looking bodysuit and a silver helmet covered with dark glass, hiding his face from view. It is revealed his face is identical to Sora's face, aside from his yellow eyes and black hair. His suit's appearance is quite similar to Riku's Dark Mode from Kingdom Hearts and Kingdom Hearts: Chain of Memories. He wields a Keyblade that is colored in red and black, also bearing two blue eyes that were previously unique to Riku's Soul Eater and Way to the Dawn.

Fighting Style
Vanitas is a skilled warrior with the Keyblade and with magic. He is seen casting thunder and blizzard magic, performing complicated acrobatics, and even rides the flying Keyblades that Master Xehanort sends out. Also, Vanitas's battle stance greatly resembles Riku's battle stance. It also seems that Vanitas can be summoned by Master Xehanort by force of will.

Trivia

 * After his first spoken line, Nomura made a joke saying that he was to voice Vanitas. Miyu Irino, who previously voiced Sora, provided the voice for Vanitas in the end.
 * Vanitas's name is derived from the Latin word for "Vanity" or "Emptiness", and was chosen both for its meaning, and that it sounded similar to "Ventus".