Gaea is the main bad guy, doing all the bad guy things—she plots, she schemes, she threatens. She has evil minions, and then her minions have minions of their own.
On the bad guy roster, Gaea's closed eyes and earth clothes aren't as creepy as the lidless eye that is Sauron in The Lord of the Rings, nor does she have as much personality as the Joker. She's more of a default antagonist like Voldemort. But still, you need a bad guy, and she works well enough.