Mastermind is power hungry, and wishes to gain full domination of the planet as he did in the past. He has a british accent and prefers to dance (he dances like a sick moose). He also has a short temper, which results that his head is engulfed in green flames which are usually extinguished by emergency water sprinklers.
At first, Mastermind used to be a humanoid wizard with a pointy beard who used have an imperium that spanded 12 continents on a planet that had 7. After that he took the form of a levitating purple skull surrounded by green flames, which seriously impaired his abilities. Last, he took posession of Yang's body, by swapping brains with Yang. His mittens also seem to be sentient, but also serve as hands once required.