The Kid with the Remote Control, but with a twist: the controlled partner or servant is pragmatic at best, and a raving loon at worst. The actual loyalty of the servant varies, but the typical example is essentially a portable version of Sealed Evil in a Can -- though a very loyal one is arguably even creepier. The servant openly admits his opinion that the best way to deal with obstacles and enemies is through complete annihilation. The kid (who is usually younger) is the moral compass who has to keep it under control (with a little help from the Restraining Bolt), and more importantly not fall victim to temptation.