He's a brain. A being from Dimension X and one of the arch nemeses of the Teenage Mutant Ninja Turtles in the comic books. The "bay window in the underwear" is a mech suit he had made. Instead of piloting it from the head...where a brain goes...he piloted it from the stomach. And he sounded like my grandmother.
He's actually not one of their enemies from the comics! At least, not the original comics. The original comics did have a race of brain creatures called the Utrom that piloted humanoid robots like this, but they were actually good guys. They got turned into the single villain Krang for the original 1987 cartoon series.
And what is Krang? Looks like somebody's intestines, peering out through a bay window they had installed in their underwear.