Villains

The Villains are a group of scoundrels from various worlds who seek to harness the power of the Heartless. They are led by Maleficent, and originally had the goal of capturing the seven Princesses of Heart and taking control of Kingdom Hearts, to rule all worlds. After their first defeat at the hands of Sora, they are much less organized, and while they occasionally cooperate on a one-on-one basis, they no longer work in concert and follow their own, smaller goals.

Each Villain is usually the boss battle for their native world.

Kingdom Hearts Birth by Sleep
After discovering the Darkness, Maleficent began to gather people for her group, starting with Pete after he was expelled from his world.

Kingdom Hearts
Maleficent, along with the organization members, Hades, Jafar, Ursula, Oogie Boogie, and Captain Hook, kidnap the Princesses of the Heart in their lair of Hollow Bastion. They observe Clayton as a possible member, but he is consumed by the darkness.

Kingdom Hearts II
After Maleficent was resurrected, she and Pete try to reunite a new group. Pete contacts a former member, Hades, and try to get Jafar back, in addition to trying to find new members, such as Barbossa or Scar. Maleficent resuscitates Oogie Boogie and collaborates with him, however, he seems to have lost respect for her.

Trivia

 * The track "Villains of a Sort" is used for most cutscenes and battles involving the Villains.