But there's more to it than just that. Giygas was really and truly evil. He let the evil consume him, until it took him over completely and he no longer could take a physical form. His evil essence was all that was left, lost and confused and unaware of his actions. The only saving grace was the Devil's Machine, which could contain Giygas' essence and control his actions. This Machine was linked to the Mani Mani Statue, the device that Giygas used to control key figures in his plans.
Unfortunately for Giygas, there was a slight problem with his plans. It was prophecized that Giygas would be defeated by a team of three boys and one girl. All of Giygas' plans would be laid to waste due to the existence of a boy named Ness. Giygas set out to destroy Ness, even trying to plant the Mani Mani statue near him in an effort to influence him earlier on. In order to make sure Ness would be good and gone, Giygas set to attack Ness in his past, when he was young. At first he must have succeeded, for when Buzz Buzz comes back from the future, he says that all is destruction and that he has come back to help guide Ness to increase his power and save everything. Lucky for Ness that Buzz Buzz came along, for on the path was a Starman Jr, probably sent by Giygas to destroy Ness. With Buzz Buzz's help, Ness defeated the Starman and began to change the future.
This set Giygas into a panic, and he reached his evil tendrils out to find a ripe mind for manipulating. What he found was Pokey, Ness' neighbour and sometimes "friend." Within Pokey Giygas found an agent to do his work among people. Through the Mani Mani Statue, Giygas commanded those holding the Statue to allow Pokey close to them. With the destruction of the Statue, Pokey fled, trying to find a way back to Giygas in the past.
Another hotly debated topic is what if Giygas had switched consciousness with Pokey? What if Giygas was in Pokey's body, and Pokey's consciousness is the swirling mass hidden in the Devil's Machine? It makes some sense, for some of the things said during the final battle certainly aren't what one would expect from the truly evil. But it was said that Giygas was no longer quite sane, so perhaps it was just a glimpse of the madness his evil brought him - his obsession with finding and destroying Ness.
You'll notice there's no pictures here. Find out what Giygas looks like for yourself. :P