Vanitas, also known as The Masked Boy (仮面の少年 Kamen no Shōnen?), is a Keyblade wielder. He is the creation and second apprentice of Master Xehanort, the embodiment of the extracted darkness from Ventus's heart and the progenitor of the Unversed. Vanitas is one of the main antagonists in Kingdom Hearts Birth by Sleep. Due to being made of pure darkness and having a tough upbringing, Vanitas is ruthless and is quick to strike without notice. He is often calm and collected, regardless of most situations. His goal is to seek salvation by uniting with Ventus to forge the χ-blade.
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 "Sora" (空?, lit. "emptiness") in Japanese.
(Source: Kingdom Hearts Wiki)