Ask-normalhumanZim

Zim 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 as a sort of exile. While there, however, Zim heard about Operation Impending Doom II, and (obviously not understanding the purpose of his previous exile) returned to Conventia in the hopes of getting an assignment. Chagrined, 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 skool 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 the episode Gaz, Taster of Pork, "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, his own race or Earth.

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 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 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.

Zim, like all Irkens, was fused at birth with a mechanical device called a PAK. Zim's PAK provides him with life
An Irken PAK. support, all the necessary nutrients for survival, and an array of different weapons and tools. These tools include: a set of four spider-like mechanical legs, an organ harvester, scanner pads, hand-held communicators, memory drive, charging cell, atmospheric processor, high tech infrared/nightvision binoculars, holographic transmitters, and four rocket boosters, as well as a sort of biomechanical rebreather mask used in no air or bad air environments. It can be used to "reactivate" an Irken after a sudden shock to the system (i.e. the wave of raw stupidity in Plague of Babies).
As documented in Eric Trueheart's scripted but uncompleted episode, The Trial, an Irken PAK contains all of a single Irken's memories, personality, and high scores. An Irken assigned to a specific task has their PAK "encoded" for that task by an Irken Control Brain - similar to formatting a hard drive.
Zim's PAK, due to his actions after Operation Impending Doom I, has him encoded as a Food Service Drone; this was part of his banishment to the Irken snacking planet Foodcourtia, and is confirmed by Frylord Sizz-Lorr during the episode The Frycook What Came From All That Space. Interestingly, the episode Career Day also shows that the job that is the most suitable for Zim is fast food services.

In The Trial, Zim's true status was determined to be "Defective". Although a Defective is supposed to be an individual with a flawed PAK, it can be seen in the episode that the process of determining an Irken's "defectiveness" is highly subjective.

In another scripted, but never completed episode by Rob Hummel, Ten Minutes to Doom, Zim's PAK is accidentally dislodged from his spine, and then promptly stolen by Dib. Zim himself begins to quickly deteriorate, and it is revealed that Irkens can only survive ten minutes without their PAK. The PAK attaches itself to Dib and begins to assert Zim's personality in Dib. Although Zim is almost dead by the time he retrieves his PAK, once he has it back on he is almost immediately rejuvenated, even floating off the ground for a second or two. As Dib states in the episode, "This device... it IS Zim. It's his brain and his life support. That means his body is just... something to carry his PAK around".

It should be noted, however, that only Irken biology is compatible with the PAK. When Dib wears the PAK the madness of Zim's personality is far more overstated than usual. Zim later states that the PAK would have destroyed Dib.