Zim

Zim is an alien from the planet Irk. Banished to Earth by his leaders, the Almighty Tallest, Zim believes it's his job to prepare our planet for the coming Invasion (Operation Impending Doom 2). The reason for his banishment is that he almost completely destroyed Irk during Operation Impending Doom 1.

Wearing an ingenious disguise, Zim tries to blend in with the hideous native inhabitants by going to school (where he's not very popular). Zim's weird plans to take over the Earth are often very funny.

Gir

Gir is Zim's malfunctioning robot. When the Tallest sent Zim on his mission (banished him), they didn't want to waste a working SIR unit on him, so they gave him a broken one, called Gir. Gir is very, very insane and very, very, very dumb. He is almost completely useless as a sidekick; he only watches tv all day, and when he has to help Zim with one of his plans, it always ends in disaster and chaos. This actually makes Gir the most hilarious character in the show, and entire fansites have been devoted to him.

Dib

Dib is Zim's archenemy. Obsessed with the paranormal, Dib didn't have any problems seeing Zim's actually an alien. The only problem is to get people to believe him.

Dib's the son of the famous Professor Membrane (Friend of all Mankind, Enemy of Santa), and the brother of Gaz (scary girl).

The Almighty Tallest

The Tallest are the leaders of the Irkan race. They are considered smarter and better, just because they're taller.

They already banished Zim before - after the failure of Operation Impending Doom 1 - to Foodcornia, but he quit as soon as he found out about Operation Impending Doom 2. The Tallest then decided to launch him into space, in the opposite direction of the Armada, in the hope he would die. Instead, he arrived on Earth.