Zim is a member of the mighty Irken race, a species of aliens whose social hierarchy is based on height. The Irken head of state is The Almighty Tallest (during the course of the show leadership of the Irken state is shared by two beings, The Almighty Tallests, who are exactly the same height). Zim was banished to the planet Foodcourtia to fry things for the remainder of his life for destroying a sizeable portion of an Irken city with a giant robot. The robot was part of Operation Impending Doom I, a major galactic conquest venture by the Irkens. Zim was overeager and activated his robot before leaving his home-planet.
Upon hearing of Operation Impending Doom II, Zim "quits" being banished and races to the Great Assigning where Irken Elite soldiers are chosen to become Invaders whose mission it is to blend in with the indigenous life forms of their assigned planet and collect intelligence. Zim pleads with the Tallests to assign him a planet, and in a desperate act to get Zim out of their hair (used figuratively -- Irkens have no visible hair), assign him to a Mystery Planet, Earth.
The show features the exploits of Zim trying to conquer the Earth while his arch-nemesis, Dib, the only person to realize he is an alien, trys to stop him.
A short, green alien from the planet Irk, whose social structure is based on height. He is trying to conquer the Earth, and is generally unsuccesful. He has a penchant for being over-dramatic, and uses ridiculously excessive and elaborate plans to conquer the planet when he obviously has the resources to do so much more efficiently, but not the knowledge. Zim's usual disguise consists of some contact lenses and a wig, leaving his green skin clearly visible. He's been known to wear other costumes as well, including an old man diguise and a bear suit.
A neurotic, stupid robot slave belonging to Zim. He is easily influenced and impossible to fully control. He is a maliciously modified SIR (Standard Information Retrieval) unit. He is incapable of focusing his attention on important things for long periods of time and often speaks with no relevant coherent thought in mind. He is often the reason Zim fails in his attempts. His normal disguise is a green dog suit. He is infatuated with Earth culture and is a very popular character.
Child in the classroom Zim chooses to infiltrate as his primary way to gather information on the planet. Obsessed with the paranormal/supernatural since an early age, Dib is often criticized by his classmates for being crazy or mentally unstable for his outlandish theories. He is the only person capable of seeing through Zim's disguise and makes it clear since his introduction that he will live to stop Zim at every turn. Fairly normal by real world standards.
Dib's creepy sister. She has a penchant for Video Games, pizza, and striking terror into the hearts of her enemies. She knows Zim is an alien, but doesn't care about his trying to take over the world because "...he's so bad at it."
World's smartest man, inventor of Super-Toast, and father of Dib and Gaz. He does not believe in aliens or "parascience". He posseses incredible technology and has his own TV show, "Probing the Membrane of Science". He hates Santa Claus.
Teacher of Dib and Zim's class. Devoid of any compassionate emotion, she is a tall, slender aged woman of unknown origin. She displays dark powers, such as the ability to materialize from the shadows.
A minor character introduced in episode 2 and rarely seen again. He is a child with the head of a 60-something year old man: very creepy. Voiced by series creater, Jhonen Vasquez.
A group of conspiracy theorists who have an astonishingly high burden of proof, at least as far as grade-school alien invaders is concerned. Dib is, of course, a member, albeit a junior one. Dib spends a fair amount of his time trying to convince the membership of the threat posed by Zim.Characters
Zim
Gir
Dib
Gaz
Prof. Membrane
Ms. Bitters
Old Kid
The Swollen Eyeball