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DIZ is really Ansem the Wise in disguise, in Chain of Memories he helps Riku and Sora get through Castle Oblivion. In Kingdom Hearts II he builds a machine that he uses to stop Xemnas from opening Kingdom Hearts. The machine does not work and it blows up taking Ansem with it.

Auron is from Kingdom Hearts II in Olympus Coliseum, he is hired by Hades to destroy Hercules. Hades is controling Auron so he has no choice, Sora later frees him and together they battle Heartless in the Underworld.   

These are the three faries that worked as a spy for Malfiecent. They later give you the Gullwing keyblade.  

In the Kingdom Hearts series Jimmy is with Sora recording all their adventures in his journal for the King and Queen of Disney Castle.  

In Kingdom Hearts I, Ansem and Maleficent were collecting the Seven Princesses of Heart:

Aurora (Sleeping Beauty)

Alice

Cinderella

Belle

Jasmine

Snow White

Kairi

They needed them to open the final keyhole in Hollow Bastion.

King Mickey is the king of Disney Castle, he is also a keyblade warrior. In Kingdom Hearts II he gives you your Master Form, and if you die during a boss battle you may get to play as Mickey and give Sora health to fight again. (You cannot be Mickey during the final fights or during the fight with Sephiroth) 

These characters can replace Donald and Goofy during the game depending what world you are in. (Riku and Auron can also chage with Donald and Goofy)

Tarzan

Aladin

Ariel

Jack Skellington

Beast

 Jack Sparrow

Mulan

 Simba