Ultra King Bowser Koopa (also Ultra Lord Bowser, Ultra King Koopa commonly in other media) is the main antagonist of the Ultra Mario series, the supreme leader of the Ultra Koopa Troop, King of the Ultra Koopas, and the arch-nemesis of Ultra Mario. He has repeatedly, kidnapped or attempted to kidnap Ultra Princess Peach and take over the Ultra Kingdom. He has on other occasions attempted to conquer the entire Ultra World, as well as the entire universe.
Since his debut, Bowser has been in nearly every Ultra Mario game, including the main series, spin-offs, and crossovers. Besides being a primary villain, Ultra Bowser has at times played an anti-villain in several Ultra Mario RPGs, with the exception of Paper Ultra Mario.