Mega Greegee is a enormous version of Greegee. Greegee becomes Mega Greegee after ingesting a Mega Mushroom. Unlike most people who have eaten a Mega Mushroom, Mega Greegee can stay huge for as long as he wants.
One day, Greegee was thinking of how he could further terrorize the people of Earth. Coincidentally, Zario was experimenting with Mega Mushrooms at the same time. At one point or another, Zario and Greegee bumped into each other, and Zario's Mega Mushroom flew out of his hands. And guess what? Due to Greegee's unusual DNA, as well as his evil powers, Greegee could control the duration and intensity of the Mega Mushroom's effect. Greegee began to use his Mega Powers to destroy cities and towns everywhere.