Zim is a main character in the series Invader Zim. He plans on ruling Earth one day, to prove to the tallest he is worthy of Invader status.
Zim and the rest of the Irkens seem to be based off the typical "little green men from outer space" stereotype. Many Irkens are about the size of a preteen child (with a few exceptions), with Zim being exceptionally short. In The Nightmare Begins, Zim is shown as being even shorter than the notoriously short Invader Skoodge. Almighty Tallest Purple tells him that "no Invader has ever been so... very small".
Zim, like most of the Irken race, has ruby-colored eyes. It's mentioned in Walk of Doom as well that Zim's eyes contain ocular implants ("Ocular implants are standard Invader issue"). Since the eyes he was born with aren't visibly different from subsequent appearances, it can be assumed that these implants are subcutaneous.
He also possesses a pair of antennae. Their exact function is unknown, but they seem to function like human eyebrows; rising and lowering to convey emotion. It is strongly implied that they function like ears, and even pick up vibrations.
Zim's House is designed to look like a normal green home which he and Gir live in. Zim put some lawn gnomes, fake blowfish and an "I heart EARTH" flag to make it look more normal. The tentacles that come out the sides steal his next door neighbors' power and might function as a waste disposal system, seeing as that's where Dib ended up in Walk of Doom. It has a toilet and a trash bin that acts as the entrance to his underground lab. The antenna on the roof serves as a means to get TV, and to contact the Almighty Tallest. The top of the house can open up for Zim's ship to fly out or land in.
Zim is, as previously noted, an Irken with megalomanical, sadistic, sociopathic and narcissistic tendencies. He usually ends up being a danger to himself and his own people. Jhonen Vasquez has said that Zim has a nanocomplex.
Despite his self-proclaimed greatness, Zim is held up as a complete laughingstock, a scapegoat, or a figure of contempt by his own species. Despite this, Zim is innocently loyal and obedient to the Tallest and his "mission" in Operation Impending Doom II, to the point where he is willing to endure pain and hardship in order to gain their respect. In the episode Planet Jackers, Zim even takes a severe beating in order to protect his mission and Irken interests.
His self-absorbed nature prevents him from seeing his failures, be they in the past or present. He claims to be proud of all the past disasters he has caused, and rarely acknowledges his defeats in his present mission on Earth (often blaming them on other characters, other factors, or by completely changing the subject). When he is told by Tak in Tak: The Hideous New Girl, that his "mission" was really an attempt to get rid of him, he simply rejects her statement as a lie. It is not clear whether Zim is aware of his failings in the eyes of his leaders and is desperately trying to compensate for them, or whether he really is just completely oblivious to the opinions of others - the series is not consistent on this point. By turns this would-be Invader can be petulant, critical, paranoid, humorous, sarcastic, pretentious, pedantic, melodramatic, and even occasionally whimsical.
He is often curiously attached to random objects, such as his robot bee (Tak: The Hideous New Girl). He is immensely proud of anything he creates, yet is often frustrated when his devices fail to live up to his grandiose expectations.
It's clear that Zim is highly intelligent (in his own crafty sort of way) and is a keen manipulator of tools and devices, but has absolutely no apparent interest to think things through to their logical conclusion. The fact that creating and unleashing a giant water balloon on the city from orbit in The Wettening would also destroy his house doesn't seem to enter his mind, and when he hijacks the Massive in Backseat Drivers from Beyond the Stars, he only assumes (incorrectly) that the Tallest will be appreciative of his efforts. Additionally, he does not stop to consider the consequences of restoring the natural time-flow of a city-leveling explosion in Walk for Your Lives, despite being warned several times, by GIR, no less, until his base was coming down around him. An alternative explanation is that he is aware of the consequences of his actions, but believes that they are worth reaching his goals and is willing to amend them after he does so.
Zim clearly is more than capable of destroying Earth, but he hardly ever uses this capability correctly. For instance, when he is accidentally sent a Megadoomer robot, he decides to use it to kill Dib, when the robot obviously has the destructive power to do better things. He has often come very close to taking over the planet, only to be thwarted by Dib or occasionally Gaz. One of Zim's apparent skills are his somewhat inconsistent skills at piloting; many episodes feature a dogfight or tense aerial situation, although just as many show Zim horrifically failing at piloting skills as well (ie: He often crashes into the roof of his house before landing perfectly on the airpad). And, although superior in magnitude to the skills of enemies like Dib, Zim's piloting skills pale in comparison to enemies adept at hand eye coordination or overall agility (such as Gaz or Tak). He also seems to have incredible engineering ability. In Walk of Doom, he improves upon the SIR unit guidance system chip (although how GIR got one in the first place is unknown), and in Walk for Your Lives, he single-handedly creates a variable time stasis field, capable of altering individual time-fields.
Although Zim often states he wishes to enslave Earth, it seems unlikely that he would actually want to rule it. In Attack of the Saucer Morons, Zim gains a troupe of humans who would be willing to do his bidding. Zim, however, panics and tries to escape from them, believing that they are trying to capture him for dissection and study. In the Nickelodeon Magazine Special, in which Zim finally takes over the world, he doesn't stay to rule for very long, instead leaving Earth forever (along with Dib, GIR and Minimoose) to ride alien bunny rabbits and drink soda. In an interview with Animation World Magazine, Jhonen Vasquez has confessed that the Irkens would really have no desire to enslave the humans, and would much more likely exterminate all Earthly life in order to use the space to build another parking structure planet.
Zim shows no remorse for people that are harmed due to his actions and almost never takes responsibility for his mistakes, and prides himself on causing destruction in the name of the Irken Empire, even when said destruction is aimed towards other Irkens (as Zim responded about his destruction of Irk, "... But I blew up more than all the other Invaders!" with Tallest Red responding "You BLEW UP all the other Invaders!" Zim replies with a dismissive "Oh, hmm!". More or less he considers everyone else but himself (even members of his own race) expendable for his goals, often exploiting them to reach his own goals with apparent disregard for their safety or well-being. For example, in Hobo 13, Zim sends the soldiers under his command to their dooms when other, safer methods could be taken (even using Throbulator, the last soldier in his unit, as a battering ram), causing Sergeant Hobo 678 to fail him automatically despite the fact that Zim manages to survive the obstacle course. Another example is when Zim destroys a candy machine in Tak: The Hideous New Girl, ending Tak's mission to become an Invader. Additionally, he fails to save her when she is trapped in the assessment room (although it is implied that he simply didn't notice her inside). His unawareness of his actions are most likely the result of him being a "defect" as seen in the cancelled episode The Trial.
Zim usually shows little to no affection, often claiming to "need no one", but he occasionally lets a softer side of him slip (probably by accident). In Parent Teacher Night, when Zim thinks back to when he was born, he remembers that the first thing he ever said was that he loved the "cold, unfeeling robot arm" which hatched him from his test tube and activated his PAK. This memory triggered him to sub-consciously hug Ms. Bitters in the same way that he hugged the arm when he was a smeet. In addition, while he frequently yells at GIR and orders him around, he seems to have taken a liking to the robot, even if he doesn't want to admit it. In the episode Walk of Doom, after seemingly reducing GIR to tears with a harsh reprimand (although GIR was only crying over the fact he ate his cupcake), Zim appears remorseful and quickly forgives the robot, also acknowledging that his feelings would not help them, stating "I can see that you understand your mistake, GIR, and me being angry will get us no closer to home". Additionally, in Halloween Spectacular of Spooky Doom, Zim expresses concern for him when he leaves him behind upon entering the Nightmare World, stating that he "only hope[s] poor GIR is all right by himself against those zombies". He probably views GIR as a little brother of sorts, since he constantly has to reprimand him and get him out of trouble.
In order to blend in with the inhabitants of Earth, Zim has made a series of creative disguises. Zim often wears disguises to attempt to conceal his identity from the "earth-monkeys" that he plans to eventually conquer; they are often unconvincing and hastily constructed, but he knows that the humans are too stupid to recognize that he is an extraterrestrial visitor anyway.
Zim's school disguise consists of some contact lenses and a bouffant wig. The contacts give his eyes a human appearance, and the wig covers his antennae; however, everything else about him, including his uniform and green skin, is left unchanged. He explains away his skin color and lack of ears as a skin condition, (The Nightmare Begins), and has passed off being caught without his contact lenses as a bad case of pinkeye. It was discovered in the first episode that the sleeve of his right glove can become a self destruct device, most likely to get rid of an Irken soldier if they believe their mission has been compromised. He occasionally wears an old man disguise in Walk of Doom and in other episodes, which he uses throughout the first season. This disguise consists of a flowered hat and a white fake beard. It makes him resemble a short, crazy old man, and in Walk of Doom, he was mistaken for a hobo. In the second season of the show Zim adds another disguise to his arsenal: a floppy, huge-headed human suit with a cape, a sewed-on handbag and stuffed puppy, and a name tag reading 'human'.
Other costumes worn include a fat lady outfit, used to hide the time field explosion (Walk for Your Lives), bear suit (use unknown, possibly an I Feel Sick reference) (Battle of the Planets), 'Baby Inspector' disguise for interrogating 'Noogums' (Plague of Babies) and the Santa suit and Easter Platypus disguises used to manipulate the public in The Most Horrible X-mas Ever. The Pilot also has an oversize robotic battle-suit shaped like Zim's school disguise for use in the food-fight that occurs.
Brief on-screen appearances of other Invaders in Walk for Your Lives and Planet Jackers, show that almost all the Invaders (a notable exception being Tak, who uses a clever hologram) are using even shoddier disguises, so Zim's ineptitude in this regard is not unique.
Zim is severely allergic to several Earth substances, including polluted water, meat, barbecue sauce, beans, pizza and most skool lunches. The last item may not be considered a weakness exclusively to Zim, as the food in the cafeteria has been shown to be harmful to humans as well. Exposure to excessive grease leads to the creation of enormous transparent, semi-permeable, hypnotic pimples, as seen in Rise of the Zitboy.
It's unclear whether these effects are due to Zim's alien biochemistry or because the Earth itself appears to be, in the time-frame of Invader Zim, excessively polluted. The episode Bolognius Maximus offers an additional suggestion that the Irken body may be physiologically and biochemically much more delicate than a human's - Zim's genetic structure breaks down and turns into bologna much faster than Dib's after exposure to an identical infusion of bologna-DNA.
However, for each weakness Zim has found a compensatory mechanism - he discovered that bathing in white school glue repelled water (The Wettening), and that germs could be repelled by covering his body in "space meat" (which is actually made with napkins) and using conventional Earth disinfectant sprays, despite the meat then fusing to his flesh (Germs). He also discovered his pimples have hypnotic properties over humans. In the episode Zim Eats Waffles, he thinks that eating enough Earth food will allow him to build up an immunity to it. Whether it has or not wasn't seen, but since the food was made with mainly nonfood ingredients (GIR at one point announces "These got peanuts and soap in 'em!"), it probably would not have made much of a difference.
Although Zim has severe burn-like reactions to Earthly meat and polluted water, his skin seems to be strangely fireproof. When Zim comes in contact with open flame, his skin does not burn or appear to react at all. Examples of his resistance to fire are in Battle of the Planets when he catches fire while piloting a flaming Mars and during a cut scene in Abducted when he falls through a ring of fire and comes out unscathed (if not a little shaken). He is completely unconcerned during both experiences, and even responds with a nonchalant "Oh, well" when GIR tells him he is "on fire" in Tak: The Hideous New Girl.
In addition, after Zim found out about bathing in the school glue from The Wettening, it appears that he developed a layer of glue that permanently fused to his outer epidermal layer, and it protects him against water. It is first seen in Door to Door that Zim can withstand a large amount of water (when the character who Zim approaches sprays him with a hose). This was later seen again in The Frycook What Came From All That Space, when Zim crashes and has a small splash of water from a hose. In the DVD, Vasquez commentates that "Zim is resistant to water, but he's exposed to polluted water only".
Zim also has an intense fear of germs and anything "filthy", and can be described as acutely germophobic. In the episode Germs, Zim orders a pair of "micro-goggles" that allow him to see germs, portrayed in the usual cartoonish fashion of large, pulsating green blobs. Zim panics when he realizes that germs are almost everywhere and begins to spray them with disinfectant. In Rise of the Zitboy, Zim obsessively rubs his face with soap after being hugged by a grease-soaked GIR. He also has similar reactions to "filth" in other episodes, and it is a running theme in all three seasons.
An unknown substance secreted by his skin kills lice almost instantly, as seen in Lice.