Zeromus. The esscence of all that is evil. Known as one of the most powerful final bosses ever. Known for not playing any role at all in his own game until the very end when he suddenly appears.

...ok, so that can apply to a lot of Villains in the Duelling League, but that's not the point. With excellent speed and his trademark Big Bang, backed up by his Flare counters against magic, Zeromus is a not a foe to be trifled with.