King Greg is the King of Atlantis, which was sunk beneath the sea by Cosmo many years ago.
He is a slightly fat merman with white hair and beard, golden gauntlets and earrings and a green tail. He is able to slide on land with his tail. Like most of his species he has dwarf/goblin ears instead of normal human ears.
Calling Cosmo the "Accursed One", he sentences Timmy, Cosmo and Wanda to be eaten by a Kraken unless Timmy can prove why sinking them was a good thing, as life underwater is better than living on land.
He is eventually convinced when Timmy shows him the movie "Wet Willie", where the movie quality and concession prices scare him and send him and his Krakens fleeing back to the sea. In the end, he pardons Cosmo, but a giant squid summoned by Timmy earlier in the episode attacks Atlantis and he brands Timmy as the new "Accursed One".