Vanitas

Vanitas (ヴァニタス, Vanitasu) is the Darkness from Ven's Heart given physical form, created by Master Xehanort. His name Vanitas means "emptiness" in Latin, but also suggests "vanity" and it was also stated by Tetsuya Nomura that in Japanese the "Vanitas" contains the kanji for "Sky" (Sora) however it meand "emptiness" instead (sky and emptiness use the same kanji). He resembles Sora for the same Reason Roxas resembles Ven, Because Ven's (literally) broken Heart connected with Sora's Heart. He originally wields a Keyblade simliar to that of Riku's Soul Eater, using a similar fighting stance and armor nearly identical to Riku's Dark Form. He monitors Ven's strength and progress in the hopes of absorbing his power prior to obtaining the Χ-Blade, a powerful Keyblade that can unlock Kingdom Hearts. By the end of the game, after being defeated within Ven's subconscious, Vanitas is destroyed and the Χ-Blade is shattered by Aqua in the physical realm. Ven's body is rendered dead while his incomplete organs enters a young Sora. He is voiced by Miyu Irino in the Japanese version of the game.

Kingdom Hearts Birth by Sleep
Four years before the events of Kingdom Hearts Birth by Sleep, Ventus served as Master Xehanort's apprentice. The Keyblade wielder was sent to the Keyblade Graveyard, where he was forced by his master to face several Heartless. Ventus lost the battle, and was knocked unconscious. Master Xehanort then proceeded to use his Keyblade to unlock Ventus's heart, extracting the darkness within and resulting in an orb of dark energy forming into a young boy in a mask who is then given the name "Vanitas" by Master Xehanort. The dark enigma is also the source of the Unversed, as well as a key component in Master Xehanort's plans.

After Master Xehanort views the battle between Terra and Aqua as part of their Mark of Mastery, he leaves the room to find a hidden Vanitas awaiting him. The two converse about Ventus, and Vanitas claims that Ventus's performance was weak, and he still needs to "break that loser in." Master Xehanort tells him to do so somewhere else as he needed to keep up appearances. So Vanitas decides to give Ventus some "incentive." Vanitas later appears before Ventus in his room, speaking of Terra being a different person, and that Ventus should follow him to see it. Ventus denies his dark half's claims, saying that Vanitas does not know the first thing about Terra, and that they will always be friends. Vanitas simply mocks his friendship with Terra, telling him to grow up and vanishes into a corridor of darkness, claiming that unless Ventus sees it for himself and looks beyond his "own little world", he will not know the truth.

After his first few ventures throughout the worlds, Ventus spots Vanitas in space and is lured to the Keyblade Graveyard. Ventus asks him what he meant about Terra being a different person. Vanitas simply reiterates what he said before, that Terra will be gone forever, and challenges him to a duel, intent on testing his power and revealing for the first time he is a Keyblade wielder. Vanitas gains the upper hand mid-way, knocking Ventus down. Vanitas then prepares to eliminate Ventus for being at such a low level of power, deeming the boy too worthless to be of use, but before he could do so, Mickey Mouse appears, healing Ventus. Mickey Mouse and Ventus then proceed to fight Vanitas, who simply shrugs off the fight and leaves yet again upon his defeat, claiming that Ventus is on "probation".

After the battle against the Trinity Armor in Radiant Garden, Vanitas appears to Aqua and approaches her, taunting her by inquiring about Ventus and if he has gotten stronger. When Aqua asks what he means, Vanitas simply states that he is asking the questions as he claims he will be the only one who will walk away alive, and attacks, the two fighting a vicious battle. After the duel, Aqua attempts to remove Vanitas's helmet, but the dark enigma startles her with an insane laugh. He then compliments her skill, making an ambiguous comment about her being his backup plan, revealing that it was all a ruse to test her strength. Ventus overhears Vanitas's last few words to Aqua, returning to her side just as he leaves in another corridor of darkness.

While in Neverland, Vanitas encounters and kidnaps Mickey Mouse on Master Xehanort's orders. Though the fight is not shown, it is clear Vanitas won. The villain later crosses paths with Aqua again, this time with Ventus's cherished Wooden Keyblade. Vanitas cruelly mocks Aqua for playing treasure hunter with the Lost Boys and snaps the toy weapon in two, claiming that Ventus has outgrown the toy. Vanitas also claims that he has has outgrown his need for Aqua as a backup plan, and then battles her once again, intent on eliminating her, but is soundly defeated and knocked out cold. The battle leaves Aqua exhausted and enraged at the thought of Ventus being in danger, and Vanitas then disappears through a corridor of darkness.

Vanitas encounters Ventus again in Destiny Islands, after the latter is thrown through a portal summoned by Terra. Ventus initialy tells Vanitas that he wants nothing more to do with him. But Vanitas simply tells him that since they are equal in power, they can fuse together to create the χ-blade. Ventus refuses, claiming that the χ-blade cannot be created if he does not fight. Vanitas, though slightly angered, remains unfazed, responding only by coldly stating that Ventus was once "too broken to talk back." Then, in a series of flashbacks, Vanitas explains his birth from the darkness in Ventus's heart, and how he took his place as Master Xehanort's apprentice. This causes Ventus great pain in regaining his memories, while Vanitas vanished as Ventus fled the world. However, Vanitas quickly invited Ventus to the Keyblade Graveyard, where he threatens to "choke the life out of Terra and Aqua", and callously wonders how long Ventus will "play the pacifist."

When Terra, Ventus, and Aqua approach their final battle with Vanitas and Master Xehanort at the Keyblade Graveyard, intense battles ensue. The two villains meet up with the trio, and after a few heated words, Terra quickly charges towards them. Master Xehanort intervenes, however, blocking Terra's path with massive rock formations. As Terra continues to pursue Master Xehanort, Vanitas goes after Aqua and Ventus. The two fight valiantly against the dark apprentice, but not before Vanitas returns to Master Xehanort's side after Ventus is frozen. He battles Terra alongside Master Xehanort, who later commands Vanitas to go after Ventus and Aqua again.

Vanitas attacks Aqua from above, knocking her unconscious, and prepares the final blow to eliminate her while Ventus lies helpless and frozen. Ventus summons all of his willpower, however, and breaks out of the ice to face Vanitas in battle, defeating him once again. Vanitas compliments Ventus on his skill and finally reveals his face, explaining himself to be the source of all Unversed. Vanitas proudly explains that his plan was to spread the Unversed across the worlds, so Ventus could leave his home and grow strong enough to forge the χ-blade. While explaining this, Vanitas summons several Flood Unversed to restrain Ventus as he forcefully integrates into him. In a burst of light, Vanitas and Ventus are transported to the Dive to the Heart. Ventus's pillar now hosts a picture of Vanitas alongside his own. Vanitas wields an incomplete version of the χ-blade.

Vanitas claims their fusion was incomplete, and they only managed to make a fraction of the χ-blade. He then tries to fuse with Ventus once again, but Ventus refuses, vowing to defeat Vanitas and destroy the χ-blade. The dark apprentice simply laughs, confident in the fact that they both know that if the χ-blade is destroyed, so will be Ventus's heart. However, Ventus rebukes him, saying he will fight for his friends no matter what, even if the price is his heart, despite Vanitas's mocking of his reason to fight. The two Keyblade wielders then begin a final battle in the Dive to the Heart.

As Ventus battles Vanitas in their Awakening, Aqua and Mickey are forced in the Keyblade Graveyard to battle a sort of Ventus-Vanitas fusion, which is Ventus's form with Vanitas's golden eyes and bodysuit. Ventus-Vanitas summons the completed χ-blade, and exclaims with psychotic glee that he plans to unlock Kingdom Hearts so the worlds can be connected and the legendary Keyblade War can begin again, and attacks the duo. As Aqua defeats her possessed friend and destroys the χ-blade, Vanitas's metaphysical battle with Ventus comes to an end after he shatters the pillar on which they were fighting. Vanitas fades out of existence, taking all Unversed in existence with him and relinquishing his control over Ventus. Unfortunately, the victory is bittersweet, as it is soon revealed that Ventus is dying from transient apical ballooning syndrome and falls into a coma in the process.