Zim, like most of the Irken race, seems to be based on the typical "little green men from outer space" stereotype. Many Irkens are about the size of a preteen schoolchild, but Zim is at the moment exceptionally short, earning him ridicule from his fellow Irkens, who are respected only if they are tall in height rather than be respected by character, inner worth or skill. Zim was shown to be even shorter than the notoriously diminutive Skoodge. In addition to his short stature and green skin, he also has round teeth (resembling gums more than actual teeth), and a long, worm-like tongue. Zim also has ruby-colored eyes; a very common trait in Irkens and his eyes contain ocular implants. As he considers himself to still be an Invader, he continues to wear the standard uniform: a red tunic-like garment with three black stripes and a triangular pink collar and sleeves, as well as black boots, leggings, and gloves. His PAK is light gray with pink spots. He also possesses a pair of antennae, however, their exact function is unclear. His antennae visibly pick up vibrations. Zim's antennae convey emotion by either standing alert or drooping while he "schmoops". While most Irkens are expected to be ruthlessly loyal to the Empire, Zim is considered overzealous even by their standards: Brash, arrogant, and overconfident, Zim's tendency to cause wanton destruction makes him a danger to himself and his own people, yet he still considers himself a mighty Irken warrior. In addition, he is acutely narcissistic and megalomaniacal, congratulating himself on even the smallest of successes; these, however, are few and far between. However, the pride he feels for his planet and its culture, as well as its people, is to be admired and respected by all. Zim has a loud, petulant, overbearing and harsh personality. Despite his self-proclaimed greatness, Zim rarely succeeds in his attempts and is sadly held up as a complete laughingstock, a scapegoat, or a figure of contempt by his own species, a fact that he is either oblivious to, knows about but keeps a brave face, or simply chooses not to notice; this is hinted at by the fact that due to his self-absorbed nature (and the influence of his PAK), it prevents Zim from seeing his failures, past and 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 like other supervillains do, or other factors, or completely changing the subject). When he is told by Tak that his "mission" was really an attempt to get rid of him, he simply rejects her statement as a lie. It is not entirely 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, as he had deluded himself into thinking he's "amazing" and can do no wrong. Despite this, Zim is innocently loyal and obedient to the Tallest, the Irken Empire, his planet and his people, and is willing to go to ridiculous lengths to please his leaders, to the point that he is zealously willing to endure pain or hardship in order to gain their respect. By turns this would-be Invader can be petulant, critical, paranoid, humorous, sarcastic, pretentious, pedantic, depressive, melodramatic, and even occasionally whimsical. He is often curiously attached to random objects.

Dib resembles his father Professor Membrane, they have black lightning bolt-like hair. His hair spike seems to be solid, and retains its shape even when getting wet. It took a water balloon of ridiculous proportions dropped from Earth's orbit and a second normal water balloon in order to get it to bend. Dib seems to be able to zip-line with his hair spike (using the spike as the runners.) Likewise, it seems solid enough to punch through a brick wall like any part of his body if Dib is flung hard enough. He wears a pair of round glasses and a trademark black trench coat along with a blue shirt with a white ghost with a cartoon art style on it. He wears red shoes. He considers himself to be the sole defender of Earth, and rightly so; he's in fact the only one who knows (or cares) that Zim is an Irken wanting world domination and the subjugation of all humanity. When Dib met Zim, Dib was initially shown to be more antagonistic, obsessed, and desperate, making it his sole goal in life to expose Zim as an alien. However, he eventually realized that any efforts to reveal Zim would be wasted on his oblivious and mean peers, and Dib eventually lost his obsessive, desperate edge, becoming a more humorous, depressive and more sympathetic person. His hatred of Zim becomes real as the Irken takes every opportunity to make his life more miserable than it already is, making Dib's efforts to beat Zim all the more personal. Dib shares his father's scientific and technological prowess, with skills that far exceed the average human being. He has been known to make advanced technology entirely from scratch. These skills aren't limited to just Earth-based technology. Dib shown to be slightly more smart than his father.