Invader Zim is the titular character of the series of the same name. He is from Irk, a planet far out in the Milky Way galaxy. He is a short Irken defective but, despite his size, he feels that he is superior to other Irkens. A narcissistic megalomaniac by rumor and behavior, as well as speech, he also hates humans and Earthly culture (as would any Irken), and dreams of conquering the planet for use by the Irkens. He thinks the Almighty Tallest sent him on a "secret mission" to Earth, but the real reason was that they despise his existence, and want to get rid of him for good. Zim goes to Earth despite this, and upon arriving forms a deep rivalry with Dib, an avid cryptozoologist (or, on the show, a paranormal investigator) from the planet. Zim wants to conquer the Earth to prove himself great, likely more so, than for the glory of the Tallest and the empire of his homeworld, Irk.
Weaknesses and immunities
Zim's skin sizzles and smokes once polluted water touches it. The episode "The Wettening" first clearly shows this, and it has since become one of Zim's primary bodily weakness. He also hated human food (as he thought it was "human filth") until the episode "Zim Eats Waffles". Zim develops an immunity to Earth food by this time, as throughout the episode, Zim eats delicious waffles prepared by GIR throughout the episode (although GIR may have made the waffles using Irken material, which Zim is able to eat).
Overreaction and overdramaticism
He has a penchant for being overdramatic, saying phrases like "I AM ZIM!" and "Silence!", and uses ridiculously excessive and elaborate plans to conquer the planet when he obviously has the resources to do so much more efficiently. Often Zim overreacts by screaming and shouting, even just when he's speaking to GIR, his robot slave / assistant. Richard Steven Horvitz, the voice actor for Zim, later admitted on the first Invader Zim DVD's interview with the voice cast, titled "The Voices of Zim", that the constant shouting tired his voice during the recording sessions, and he would have to go home and recup "for a day, day and a half, uh, beyond that", as Horvitz himself explained. In the cancelled episode called "The Trial" (see below), it is revealed that Zim's overdramaticism and narcissistic behavior was probably caused by a corrupted data path in his Standard-Issue Irken Utility Pack, or his Pak.
Disguises for when being seen in public
Zim often wears disguises to attempt to conceal his identity from the "pig-smellies" that he has to eventually conquer. They are often poor disguises. Zim's usual disguise consists of some contact lenses and a wig resembling Pee-Wee Herman's hairdo (pictured at above right), leaving his green skin clearly visible (he falsely claims, in the first episode of the show, that it's an unnamed skin condition (most likely gangrene), and in the episode Vindicated!, that his red eyes are due to pinkeye). He's been known to wear other costumes as well, like an old man disguise, an elderly woman disguise, a bear suit (actual purpose is unknown; it was most likely a running gag originating from Jhonen Vasquez's earlier comic, I Feel Sick), a poorly-constructed parent costume, and a Santa suit made from a Vortian liquid material.
Prospective occurrences during Zim's "secret mission" on Earth
If the series were to continue, Zim would have to undergo an Existence Evaluation Trial (an evaluation, based on past crimes, to determine whether an Irken is worthy of continuing to exist), get kicked out of his base by the Parent Decoys (which are robots designed by Zim to make people think he is a normal human child), and enter GIR in a SIR unit contest. After the series ended, Jhonen wrote a four-part comic for Nickelodeon magazine, in which Zim actually did conquer the earth when Dib wasn't looking, and took Dib, GIR, and Minimoose to another planet to drink space sodas and ride giant alien bunnies. This shows that if Zim ever did conquer Earth, he probably wouldn't do anything to it. In addition, one of the sketchy plots for the series finale, Invader Dib, would have had Zim defeating Dib and Tak and becoming Irk's new Almighty Tallest soon afterwards; having teamed up with Gaz to put a stop to the fighting, he chooses her as his queen to sit on the throne beside him and rule the planet. This was approved by Jhonen himself.
Zim is currently voiced by Richard Steven Horvitz. In the pilot episode, titled Food Fight, Zim's voice was provided first by the famous Star Wars actor Mark Hamill and then by by veteran voice actor Billy West. Horvitz, however, was asked to come back, after being denied the part, to revoice Zim as a tester in the new version of the pilot, which currently is not seen on any DVD.
Zim is, as previously noted, an Irken with megalomaniacal and narcisstic tendancies, but it is interesting to note that in Horrible Holiday Cheer, Jhonen Vasquez mentions Zim acquiring a more sensitive side by the series' abrupt end, leading to his full turnaround following the events of Invader Dib.
Zim seems to regard Gir as both a 'robot slave', and his only real companion. Early, he exibits the desire to dispose of him, yet during episodes like Battle of the Planets and Halloween Spetacular of Spooky Doom, he shows concern for the robot's well-being. This is an example of dynamic characterism, or the progression of changes that a character undergoes in a work of fiction.
Zim is extremely loyal to the Irken Empire, but considering his less-than stable nature, it probably isn't a good idea to trust this point of view.
Zim is actually quite intelligent despite Gaz's hateful insults of him being "stupid"; his frequent failures are actually due to both his overzealousness and his desire for grandious plots when subtlety would be far more effective.
It should be noted that Zim does not qualify as an out-right villain; he sees what he is doing as the right thing.
One of the sketchy plots for the series finale Invader Dib would have had Zim defeating Dib and Tak and becoming Almighty Tallest soon thereafter, deposing Tallest Red and Purple from the throne; knowing that he must have a queen to rule alongside him, as every king needs one, he chooses Gaz as his queen, as he had teamed up with her after she (cloaked in a hood) encouraged him to fight on to save his people from certain death. Additionally, he would begin to lose his desire to conquer Earth and, as Jhonen claims, see the beauty in Earth life and thus decide to spare the planet from conquest by his people. These events would have shown a passionate, loving side to him as well.