The Unversed

The Unversed (アンヴァース Anvāsu?), meaning "those who were not well-versed in their own existences", are the main enemies of the game Kingdom Hearts Birth by Sleep. They are considered "the opposite of human life" and grow from the negative emotions produced when Vanitas was created from Ventus. It is because of this that Vanitas is able to control them. By all accounts, the Unversed are extensions of Vanitas himself, and part of his power and influence—effectively making him the first and original Unversed.
Chronologically, the Unversed are either the First or Second Enemy referenced during Deep Dive, as they are the second race to manifest, but the first to attack the Realm of Light.

Around the same time as Master Xehanort's leave from the Land of Departure, the existence of the Unversed is brought to the attention of Master Eraqus by the Keyblade master's old friend, Master Yen Sid. Eraqus sends two of his three apprentices, Terra and Aqua, on a quest to find Master Xehanort and stop the Unversed threat. He did not plan on his youngest apprentice, Ventus, trailing after his two friends, and it is due to Ventus's quest and his final battle with his dark side, Vanitas, that the truth behind the Unversed is revealed.
Vanitas explains that the Unversed are his creation, and since he is a human created from the purest, most raw form of darkness, he can manipulate the negative thoughts and emotions of people. He spawned them as Unversed and spread them across the worlds, so that Terra, Ventus, and Aqua can fight them off and grow stronger in the process. The sentiments of the defeated Unversed eventually return to Vanitas, and the race as a whole disappears upon Vanitas's destruction.

Several varieties of Unversed appear over the course of Kingdom Hearts Birth by Sleep. Emotion was a key component in their conception and design. Thus they are seen with smiling, angry, and crying "faces," referring to the shape of their red eyes, which all Unversed share. The emblem most Unversed are christened with is also meant to represent a heart that expresses emotion.