Zim is the main protagonist from the Nickelodeon animated series, Invader Zim.
He is a member of the alien Irken race. As Zim is severely emotionally unstable, he is considered defective. Zim, on the other hand, believes that he is a mighty warrior. Since his actions usually lead to disaster (he nearly destroyed the Irken homeworld during Operation Impending Doom I) his leaders, the Almighty Tallest sent him to Foodcourtia to a sort of exile, when Zim returned from Foodcourtia when he heard about Operation Impending Doom II (Obviously not understanding the purpose of his previous exile) the Almighty Tallest sent him on a "secret mission" to Earth with a defective SIR Unit, called GIR, in order to keep him away from Operation Impending Doom II.
After arriving on Earth, he enrolls in the nearest elementary school to begin his infiltration and becomes quick rivals with Dib, an avid preteen paranormal investigator. Zim wants to conquer the Earth, mostly to prove his superiority and impress his leaders. However, as mentioned by Gaz in one episode, "He's so bad at it".
Although he is the protagonist of a children's show, Zim is, morally and ethically, a villain (though occasionally considered an antihero), being overall a fairly unsympathetic character and usually only causing chaos and destruction for either himself or his own race.