By Zig

Zuriel is a demonic entity which came to the planet before sentient beings lived there. He has the power of foresight, where he has frequent visions of the future and he saw that one day humans and espers would come into being, and that using them he could gain indefinite power. He also gains power from hatred, sadness, and suffering. 

He begins by silently preying on primitive man, using people's grief and anger at the difficulties of life to manipulate them into hurting others by appearing to them in dreams, or luring them to secluded areas where he appears directly to them as dead ancestors. Each time they commit a violent or cruel act Zuriel's powers become a tiny bit stronger. 

As the generations moved forward and mankind spread across the world Zuriel took a human form to learn more about the people he foresaw would usher him into godhood. Over time he began to take on more and more human traits, most prominently a sense of humor and an obsession with laughter. 

Zuriel's powers continue to expand until he can cause conflict and suffering to entire villages that he visits, often becoming so enthralled with laughter that his real voice slips through the human facade. 
Zuriel sees that his greatest threat, and his greatest source of power will be the espers. After thousands of generations of wandering Zuriel enacts his plans. He influences the minds of military leaders and entire armies to forcibly capture espers in order to take their powers for use by humans. The espers retaliate violently quickly escalating into a global conflict known as the magi wars. 
Upon learning more about espers directly Zuriel is able to begin siphoning a part of their magical powers into himself, and as more are captured and the war rages out of control his powers increase exponentially. 

After both armies had diminished in strength Zuriel reveals himself. Zuriel forces humans filled with hate and bitterness from the war to serve as his personal army and many form a cult and begin to worship him. 

Once the espers realized what had happened they sealed themselves off along with the statues of the gods to prevent Zuriel from ever obtaining them. Zuriel had seen so much of what would come and everything he saw was true, but he couldn't see everything. The Lunarians Fusoya and Golbez gathered a group of pure-hearted humans to confront Zuriel. By the time he saw what was happening it was too late. His human form had left him vulnerable to the arms of death. 

He was still alive as they began to burn his body. As he smouldered he saw one final vision of the future. A nation of people, more powerful than he could have imagined, leading him to the esper world and the statues of the gods.

Zuriel again takes human form as the child Kefka. Growing up he slowly regains his memories and his original powers as Zuriel, until as a young adult he forsees Geshtahl will become the emperor and begins to slowly influence his mind to control the world using technology and to seek out and study the espers and their world. The suffering brought on by the expansion of the empire greatly increases Kefka's power throughout the course of the game, and once he learns about and collects a large amount of magicite he is once again able to become incredibly powerful. 