Greevil is the leader of the criminal group Cipher. He is the only character in the game to have the Trainer class Grand Master.
Although it seemed that Cipher was destroyed with the defeat of Evice, that was apparently not the case. Chief Sherles had come across some evidence that Evice was only in charge of Cipher's activities in Orre, and that the true leader was still at large. Indeed, five years after Evice's arrest, Cipher returned under the leadership of its Grand Master, Greevil.
Under the guise of a man named Mr. Verich, Greevil won the favor of the Sailors of Gateon Port by providing free food and drink every time he visited The Krabby Club. On one of his visits, he came across a Thug by the name of Zook who was pushing around a Trainer named Michael and his sister Jovi. After having one of his bodyguards, Ardos, defeat him in battle, Verich chastised Zook for his actions and left.
After Michael shut down the Cipher Key Lair and defeated Gorigan, Greevil appeared on a TV monitor and revealed himself as the leader of Cipher. He then taunted Michael by claiming that he would never reach his HQ on Citadark Isle.
At the very end of the game, Michael infiltrates Citadark Island and defeats all the Cipher members inside, including Greevil's bodyguards, Ardos and Eldes, then challenges Greevil himself. Greevil begins by summoning XD001, the Shadow Lugia created by Cipher to be the ultimate Shadow Pokemon, to destroy Michael, but he snags it. Greevil then fights Michael himself with a complete team of six Shadow Pokemon.
After Michael defeats or snags all his Pokémon, Greevil is mentally broken, unable to comprehend his defeat. Ardos tries to encourage Greevil to blow up Citadark Isle, but Eldes protests and claims that Greevil is going too far. Eventually, Greevil listens to Eldes' reasoning and agrees to accept defeat graciously, much to Ardos' anger. It is unknown what becomes of Greevil afterwards.
It is also revealed in the ending that Ardos and Eldes are Greevil's twin sons as well as his bodyguards.
And he is voiced by Frank Oz in the English dub and his Japanese voice is unknown.