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.

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 his first few ventures throughout the worlds, Ventus spotted Vanitas in space and was led 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, who left yet again upon his defeat.

After the fight, Mickey's Star Fragment sent Ventus and Mickey both to Radiant Garden. There he reunited with his friends to tackle a massive Unversed, but afterwards 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, but the dark enigma startled her with an insane laugh. Ventus overheard Vanitas's last few words to Aqua, returning to her side just as he left in another corridor of darkness.

Vanitas later found Aqua in Neverland as well with Ven's (formerly Terra's) cherished wooden Keyblade. The dark enigma snapped the toy weapon in two, battling Aqua once again. He left Aqua exhausted and enraged at the thought of Ventus being in danger, but 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.

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, who later commanded Vanitas to go after Ventus and Aqua again.

Vanitas attacked Aqua from above, knocking her unconscious, and prepared 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 integrated 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. They both fight for control of Ventus's body.

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 completed X-Blade, attacking. As Aqua defeated the possessed Ventus and destroyed the X-Blade, Vanitas's metaphysical battle with Ventus came to an end after they shattered the pillar, with Vanitas fading out of existence and relinquishing his control over Ventus.

Appearance
Vanitas wears a black and red organic-looking bodysuit, which is quite similar to Riku's Dark Mode from Kingdom Hearts and Kingdom Hearts: Chain of Memories. He wears a silver helmet covered with dark glass, hiding his face from view. It is revealed that his face is identical to Sora, although he possesses golden-yellow eyes, black hair, and paler skin. 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.

Personality
As an Unversed who feeds off of negative emotions and a manifestation of pure darkness, Vanitas is cruel, twisted, and quite possibly insane—simply put, he is pure evil. The lives of others hold no value to him, and he would kill without a moment's thought. He also possesses a high amount of pride, believing himself superior to others in every way, and is not above betraying his own master if the situation calls for it. This is in stark contrast to Ventus and Sora and other beings connected to their hearts such as Roxas and Xion, who are all kind, gentle, and valiant people who cherish their friends and possess overall heroic qualities.

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".