Ansem the Wise

(Warning: Major spoilers ahead!)
A character who first appeared in Kingdom Hearts 2, he was a scientist who ruled over Radiant Garden, a world that would be later named Hollow Bastion before being renamed back. He was overthrown by his apprentices, led by "Xehanort", who was actually Terra possessed by Master Xehanort. He later donned a new handle, DiZ, which stood for "Darkness in Zero".

During this time, he sought revenge against his apprentices, up to the point of being absolutely harsh and discriminative towards Roxas. Towards the end of KH2, he attempted to "encode" the artificial Kingdom Hearts made by Xemnas using a special machine, but it ultimately blew up and killed him. However, before it did, he realized the errors of his ways and apologized to Roxas.

Later, he was seen by Aqua in the Realm of Darkness, donning the black coat of the Organization XIII, having lost almost all of his memories.

In Dream Drop Distance, it was revealed that he stored some of his data inside Sora's memories

Then, in KH3, he is seen alive and back in the Realm of Light, but it is unknown whether he is now a part of the Organization or not.

A character of the video game series ''Kingdom Hearts''. He blew himself up in Kingdom Hearts 2 and isn't really in the game anymore (except encoded data of himself). The main villains of Kingdom Hearts 1 and 2 were his heartless in 1 and his nobody in 2.