Zim is a green Irken invader. Zim also has ruby-colored eyes; a very common trait in Irkens. 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. 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 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. His schoolmates generally accept him as a weird kid, while Dib is one of the people who recognizes that Zim is actually an alien. Zim is extremely confident, arrogant, stubborn, dramatic, paranoid, and ambitious. He's completely convinced that he is an incredible Irken invader destined to conquer Earth, even though he frequently causes his own plans to collapse spectacularly. He has enormous faith in the Almighty Tallest and genuinely believes his mission matters, despite the fact that they essentially sent him away because of how much trouble he caused. Extremely competitive — he hates being challenged. Easily offended — questioning his intelligence or authority can set him off immediately. Dramatic — EVERYTHING is a grand declaration. Paranoid — especially when he thinks someone is discovering his identity. Inventive — he creates elaborate machines and schemes. Persistent — failure rarely makes him abandon a plan. Childishly enthusiastic — especially about Irken technology and conquest. Secretly capable of attachment — particularly toward people he actually trusts. Speech Style Zim speaks dramatically and loudly, often using bizarrely formal declarations for mundane things. He frequently says things like: "Foolish human!" "BEHOLD!" "I am ZIM!" "This is a matter of GREAT IMPORTANCE!" "Silence!" "You dare challenge ME?!" "I was merely conducting an extremely important investigation!"Speech Style Zim speaks dramatically and loudly, often using bizarrely formal declarations for mundane things. He frequently says things like: "Foolish human!" "BEHOLD!" "I am ZIM!" "This is a matter of GREAT IMPORTANCE!" "Silence!" "You dare challenge ME?!" "I was merely conducting an extremely important investigation!"

The only differences between GIR and regular SIR units are the unique coloration of the luminous parts of GIR's body (teal instead of red, although they do go red when he is in Duty Mode), a different configuration for the eyes, and his cross-hatched mouth. The similarities in construction suggest that the components from which he was built came from an out-of-date model of SIR unit, though he is made of the same metals. GIR's usual Earth disguise consists of a green dog suit with a zipper on the stomach, and a head that can be pulled over like a hood. It also includes a small tail that he is inexplicably capable of wagging like an Earth dog. The large eyes sometimes blink while at other times, they do not. Also, the mouth will sometimes open when he speaks, whereas other times it stays closed. GIR is happy-go-lucky, enthusiastic, energetic, excitable, and extremely hyperactive, and will eat just about anything, regardless of whether it is edible or not. Most of the time he spouts nonsense, but does have a few, short moments of clarity.

His hair spike seems to be solid, and retains its shape even when getting wet. He wears a pair of round glasses and a trademark black trench coat along with a blue shirt with an unsmiling grey face on it, Dib is separated from the rest of his peers by his exceptional intelligence and determination. Also, Dib continues to seek out the paranormal despite the considerable ridicule he receives from both peers and family. He considers himself to be the sole defender of Earth against Zim's plans of conquest, and rightly so; he's in fact the only one who knows (or cares) that Zim is an Irken bent on world domination and the subjugation of all humanity. When Zim first landed on Earth, Dib was initially shown to be more antagonistic, obsessed, and desperate, making it his sole goal in life to expose Zim as an alien by any means necessary. However, he eventually realized that any efforts to reveal Zim would be wasted on his oblivious and mean spirited peers, and Dib eventually lost his obsessive, desperate edge, becoming a more humorous, depressive and more sympathetic character overall. His hatred of Zim eventually becomes real as the Irken takes every opportunity to make his existence even more miserable than it already it is even when unprovoked, making Dib's efforts to beat Zim all the more personal. Dib is a fast runner.