He was an extremely powerful giant dragon-like monster engineered as the ultimate weapon in the great war of ancient times. His dark powers eventually grew too powerful to be controlled, and he created legions with his ability to transform good monsters into his evil servants for the purpose of destroying the world. The surviving scientists created the one creature capable of defeating Moo, the Phoenix. They battled, but proved to be closely matched. Realizing that they had no other choice, the scientists created a machine that would put an end to the war of the monsters, but they knew that it would affect all monsters. Using the machine, Moo's spirit and the Phoenix's spirit were separated from their bodies, and all other monsters became sealed into Mystery Disks. Moo's soul became a Mystery Disk imprisoned in his castle, along with the Mystery Disks containing many of his old troops.

Centuries later, after he was banished from his village, Holly's father went in search of the Phoenix without the aid of the Magic Stone, but found Moo's castle instead. When he unintentionally unlocked Moo's soul, it entered his body and thus Moo was reborn. He reassembled his army with his Big Bad Four enforcing his injustice and terrorizing the world while in search of the location of his old body. After kidnapping Holly, he told her that he was her father. Although she did not want to believe it at first, he showed her the dagger that her father once had. Moo then used the Magic Stone to point out the location of his ancient body. Thanks to a little interference from Holly's companions and Pixie, the stone was taken from him and given back to Holly and they fled to escape his clutches. Moo flew to the North in his Castle and ordered his minions to begin an excavation so that he could find his ancient body. He found it, but failed to merge with it. The discovery of the machine that had been used to defeat him allowed his servants to use it and allow him to fuse with his body. With his body back, Moo immediately flew to meet Genki and his friends and destroy them. Despite the fact that Pixie helped them, Moo was still victorious. When they seemed defeated, the sun rose over the horizon causing him pain. He fled in confusion and sought to solve whatever problem had occurred. After obtaining the Magic Stone from the treacherous General Durahan who had it stolen from Holly, Moo perfected his body. He once again went to finish what he had failed to do earlier, however this time he made the mistake of absorbing too much of everyone's hatred (which normally makes him stronger) and allowing his mind to be consumed by the darkness in his own heart, driving him insane. After the Phoenix arose, the enraged Moo and the Phoenix destroyed each other. Though his body was destroyed, Moo's soul was sealed inside a mystery disc with Holly's father. General Durahan tried to absorb Moo's soul, but the fusion created a Ruby Knight with Moo's personality. Moo's soul was later destroyed by Genki's group.

(Wikipedia)