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, 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.
Zim and the rest of the Irkens seemed to be based off the typical "little green men from outer space" design. Many Irkens are about the size of a elementary school 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 magenta eyes. It's mentioned in Walk of Doom as well that Zim's eyes are, or contain, ocular implants ("Ocular implants are standard Invader issue"). Since he was born with two functional eyes in his head, it should be assumed that the implants were inserted later.
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. 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 All Mighty 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.
Zim is held up as a complete laughing-stock, 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, priding 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 casual "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). 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. And 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.