Starro was a crooked real estate developer put on a tiny starfish costume to scare people away from a property he could then sell... for thousands! The intervention of the Justice League of America caused him to have to expand his field of interest, and the entire Earth was almost destroyed. Starro is capable of controlling people's minds with the greatest incentive known to man. The promise of... free t-shirts! He usually gets beaten when people realize he's just a starfish and simply flick him off their faces. Usually Batman, so he has something to do while they're fighting the cosmic menace that logically should be way out of his league.
Although sometimes he seems to be a gigantic starfish larger than the Earth itself, for some reason he still can't take over the Earth until every single unimportant pedestrian has a cute evil starfish on their face.