“It happened when you and I were split into two. The negativity took shape as these monsters. They are what I feel—a horde of fledgling emotions under my control.”
―Vanitas, explaining the existence of the Unversed to Ventus[src]
The Unversed are the main enemies of 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.
The Unversed were created after Vanitas was created by Xehanort from the extracted darkness in Ventus' heart. The first few came into existence out of mixed feelings of self-hatred Vanitas felt over himself. As Vanitas tried to destroy them only to discover that he could only re-absorb them, the negativity of the defeated Unversed worsens Vanitas' mental state as more Unversed are created in a vicious cycle that lasted until Ventus came in contact with Sora's heart. Unable to defeat the very monsters that he spawned without more pain and suffering falling upon him, Vanitas finally broke down and decided to aid Xehanort to create χ-blade so his pain could ceased to be, using the Unversed as a means to do so.
Unversed, like the Heartless and the Nobodies, have a multitude of forms that are encountered over the course of Kingdom Hearts Birth by Sleep, but most are designated with red eyes and the Unversed emblem. Emotion was a key component in their conception and design. Thus they are seen with smiling, angry, and crying "faces". The emblem most Unversed are christened with is meant to represent a heart that expresses emotion.
After the Mark of Mastery conducted by Eraqus, Vanitas unleashes the Unversed on the worlds as part of Xehanort's plan to get both Terra and Ventus to leave the Land of Departure. Across the worlds, the Unversed appear before and aid those with negative thoughts and emotions. Ultimately, prior to fusing back into Ventus' body as he has some of his extensions hold him at bay, Vanitas reveals the Unversed's origins and that their primary purpose was to ensure that Ventus would grow stronger with every confrontation. With Vanitas back in Ventus' body, the Unversed ceased to be.
Unlike Heartless bosses, whom are enveloped in light when defeated, the Unversed bosses are consumed in a dark flame when vanquished.