An android. A... sentient robot. It has artificial intelligence and can think on its own.
It could also change into any shape the Master wanted because the thing about the android--Kamelion, he was called--is that he was highly susceptible to psychokinetic influence. Telepathy, in other words. And the Master was always rather good at that sort of thing...
He turned up around then, the Master. Went on about his plans and his want for universal domination... he's quite mad. Brilliant, clever, a genius... but megalomaniac to boot.