Nobody and Heartless

Nobody:
A Nobody is what remains of those who have lost their hearts to Darkness. The first generic Nobodies are seen in Kingdom Hearts II; although Xemnas appeared as early as Kingdom Hearts Final Mix, it was unknown that he was a Nobody. They are made of body and soul.
All Nobody types are named after job classes in the Final Fantasy video game series, except for Dusks, Creepers, the Twilight Thorn, and several fought with the Gummi Ship

Heartless:

The Heartless are beings of darkness that manifest in two forms, "Pureblood" and "Emblem", the second being created artificially. While most Heartless are in fact manifested hearts, they behave entirely devoid of emotion, and thus were named "Heartless".

Ienzo (Ansem's Disciple)

Ienzo was one of the six apprentices of Ansem the Wise who made his debut in Kingdom Hearts Birth by Sleep and Kingdom Hearts Birth by Sleep Final Mix. Ienzo is the original persona of Zexion; his existence continued on as Zexion after his fellow apprentice, Xehanort, led him to his gradual descent into darkness. Ienzo is also mentioned in the Secret Ansem Reports found in Kingdom Hearts II.

Story: Ienzo appears in Kingdom Hearts Birth by Sleep as a minor character with no speaking lines. His parents passed away for an unknown reason. Disobeying the orders of his master to stay inside the castle, Ienzo was ultimately surrounded and attacked by Unversed until Ventus comes to his rescue.
Shortly after the battle, his escort Even arrives and gives Ventus both his and Ienzo's thanks, explains Ienzo's past and that he had been assigned to take care of him since the death of his parents, gives Ventus the directions to the Castle Garden where he would find Terra, and returns to the castle with Ienzo.

Aeleus (Ansem's Disciple)

Aeleus, misspelled as "Eleus" in the North American version of Kingdom Hearts II, is one of the six apprentices to Ansem the Wise, and the original persona of Lexaeus. He appears in Kingdom Hearts Birth by Sleep and Kingdom Hearts Birth by Sleep Final Mix.

Story: Aeleus is shown guarding the palace of Ansem the Wise. He and Dilan meet Ventus, but keep him from getting inside the castle, telling him to leave before the monsters come out. Aeleus then spots a giant Unversed, and both guards begin to head after it as Ventus runs ahead of them.
As the two move to follow him, Even approaches and openly wonders what will become of the castle if they left their posts. Aeleus begins to argue, but Even assures Ventus could handle it because he is "special". As Aeleus questions this, Dilan recalls their job to guard the castle, and the two return to their positions at the door.
Sometime after Xehanort's defeat, Aeleus is present when Dilan throws Lea and Isa out of the castle.

Even (Ansem's Disciple)

Even was one of the six apprentices to Ansem the Wise, and the original persona of Vexen. He appears in Kingdom Hearts Birth by Sleep and Kingdom Hearts Birth by Sleep Final Mix.

Story: Even first appears as he approaches Dilan and Aeleus, and openly wonders what will become of the castle if they left their posts. Aeleus begins to argue, wanting to chase after Ventus and an Unversed, but Even assures them that Ventus could handle it, because he is "special".
As Aeleus questions this, Dilan recalls their job to guard the castle, and the two return to their positions at the door, and Even stands in place as he looks on at Ventus trailing an Unversed. He takes note that Ventus has no darkness in his heart, and after commenting that it may have been removed, he deems it impossible and turns to walk back to the castle.
Later on, Even looks around for Ienzo, calling his name. He finds him walking towards Ventus, and he approaches the two, reminding Ienzo not to leave because it is dangerous. He then turns to Ventus and thanks him, explaining that Ienzo's parents have passed away, and that he is in charge of raising him. Ventus then asks if Even has seen his friend, to which Even replies he had seen him in the castle garden. As Even and Ienzo are about to leave, Even states his belief that they are fated to meet again someday.