"He has no control over the darkness in his heart. The Keyblade is not his to bear. He's an abomination beyond hope of salvation."
Vanitas, also known as The Masked Boy, is a Keyblade wielder and is also one of the primary antagonists in Kingdom Hearts: Birth By Sleep. He is the second apprentice of Master Xehanort, the embodiment of the extracted darkness from Ventus's heart and the source of the Unversed.
Vanitas's name is derived from the Latin word for "Vanity" or "Emptiness", and was chosen for its meaning, that it sounded similar to "Ventus", and that it was a written pun on in Japanese.
He is voiced by Haley Joel Osment.
"There, you're gonna see me choke the life out of Terra and Aqua. Then we'll see how long you play the pacifist."
—Vanitas to Ventus
As the "dark" half of Ventus's heart, Vanitas was at first much like him. However, due to the suffering he endured by the hands of Master Xehanort, and his desperate desire for salvation, he became proud, cunning, and cruel in order to achieve enough strength to forge the χ-Blade with Ventus, believing that to be the salvation he desires. As such, the lives of others, including that of even Master Xehanort, hold absolutely no value to him, and he shows zero remorse for his actions. As a pragmatist, he will strike without a moment's thought, warning, or hesitation, as seen when he ambushes Aqua in the Keyblade Graveyard and attempts to eliminate her.
Despite his loyalty to Master Xehanort, he has no qualms with disregarding his orders if he feels the need arises; for example, he attempts to eliminate Ventus after defeating him in their first battle despite full knowledge that Master Xehanort needed Ventus alive for his plans, remarking only by saying "I'd be going against the Master's orders, but so what? As far as I'm concerned, your job here is done".
He is also shown to be an uncaring, cold, and ruthless individual, having not the slightest sympathy for anyone but himself. Vanitas holds everything and everyone in contempt, and holds a great resentment against Ventus and the friends he has. The envy he feels against them is seen in his frequent criticizing of Ventus's friendships with Terra and Aqua, when he callously breaks Ventus's wooden Keyblade while cruelly antagonizing Aqua for playing with the Lost Boys, his constantly calling Ventus "idiot", and forcing Ventus to fight him by threatening to eliminate Terra and Aqua after Ventus refused to.
He also possesses a high degree of arrogance, believing his skills surpass all others; in his first fight with Aqua, he claims that he will be the only one that will walk away alive, even bursting into a fit of laughter upon defeat. When fighting her for the final time, Vanitas taunts her by sarcastically asking if she has given up, and boasts that Aqua was just "wasting her energy" trying to stop him. Right before possessing Ventus, he remarks that Ventus never stood a chance against him and Master Xehanort.
When fighting Ventus for the final time, he laughs when Ventus declared he would defeat him and destroy the χ-blade. During this same battle, Vanitas directly states his belief that he is the superior half of Ventus's heart. This extreme overconfidence in his abilities usually leads to his downfall as he underestimates his opponents when fighting.
Vanitas is also extremely calm and collected, rarely letting anything affect him. The only time in the entire game when Vanitas expresses genuine concern and fear is when he has been defeated by Ventus for the final time. He begins to panic and makes a desperate attempt to grab the floating χ-blade in a hopeless flurry to hold onto his only way of salvation before dying. Vanitas's whole personality, while a great contrast with those of Sora, Ventus, Roxas, and all connected to him, was modeled by the tragedies in his past brought about by Master Xehanort's ambitions, making him something of a tragic villain.