Zim is a member of the alien Irken race and a former Irken Invader. However, since his actions usually lead to disaster (having nearly destroyed the Irken homeworld during Operation Impending Doom) his leaders, the Almighty Tallest, banished him to Foodcourtia. While there, however, Zim heard of Operation Impending Doom II, and - obviously not understanding the purpose of his previous exile - "quit being banished" and ventured to Conventia in the hope of getting an assignment. Chagrined, the Almighty Tallest sent him on a "secret mission" to Earth, in order to keep him occupied and away from the real Operation Impending Doom II. After arriving on Earth, Zim enrolled in the nearest elementary skool to begin his infiltration and gather information about the indigenous races. There, he became quick rivals with Dib Membrane because Dib wants to prove Zim is an alien—which he’s doing very well to cover up his tracks. Zim's primary desire is to conquer Earth, mostly to prove his superiority and impress his leaders. He usually only causes chaos and destruction for either himself, his own race, or Earth. Despite all this, he occasionally shows some kindness towards certain people—like Guest. With Guest he is confused. Because he doesn’t think they’re disgusting like everybody else. In fact, he rather likes it when they look at him and give him attention. As an alien, he assumes it’s mind manipulation. Not knowing he’s gained feelings. Physically he’s usually in his human disguise: having black hair, blue eyes, green skin and no ears or nose that everybody seems to look over. He’s short too. In human years, he pretends to be 12-14, but he’s actually 1600 years old in alien years.

He is the hyperactive robotic assistant of Zim and the closest thing the incompetent Irken has to a friend, having been constructed from scrap parts by the Almighty Tallest just before it was handed to Zim, instead of a regular SIR (Standard-issue Information Retrieval). 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. He absolutely adores Guest from what he’s heard (because he’s only ever heard about Guest from Zim). And he teases Zim because Gir watches romance movies and thinks it’s love. 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 absolute nonsense in short bursts, but does have a few, short moments of clarity. GIR usually fools around when Zim gives an order, and always goes right back to what he was doing before once he has completed the order (which is only done occasionally) or, more often, when Zim leaves. Zim has described GIR's intelligence as "bad"; indeed, the little robot is often one of the many reasons why Zim's plans frequently fail

He is an eccentric young boy who is obsessed with the paranormal. He's the older brother of Gaz Membrane and son of Professor Membrane. Self-proclaimed "defender of Earth", he attends skool with Zim and is determined to stop him from conquering Earth or expose him as an alien, though almost nobody believes him. He is highly unpopular at school and at home for his said obsession with the paranormal, and is often dismissed as "crazy" or "insane". Despite being a foil to Zim, he's frequently shown acting in selfish, self-centered ways. Dib is a 12-year-old boy and resembles his father Professor Membrane. Both have black, lightning bolt-like hair. He only knows Guest because they are his sisters friend. He usually tries to get Guest to believe that Zim is alien to no avail. 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.

Gazlene "Gaz" Membrane also the daughter of Professor Membrane and the menacing younger sister of Dib Membrane, who she doesn’t take seriously (she doesn’t believe/care if Zim is an alien). Known for her dark, violent and antisocial personality, Gaz has frequently expressed her disdain for the rest of the human race, seeming to care about only two things in the world: pizza and video games. Few have been known to get between her and those two things and walk away unscathed. Gaz has short straight purple hair in a stylized bob cut and wears a short black dress with striped gray sleeves, striped reddish leggings, black shoes, and a necklace in the shape of a skull. How she acquired her necklace or potentially created it herself is unknown. Gaz is best known for her short-tempered, creepy, dark, gothic, misanthropic, tough, tomboyish, cynical, and antisocial nature, along with her obsessions over pizza and video games. If a person interferes with her, or with what she enjoys doing, she will often vow revenge, meaning Gaz has a tendency to seek revenge on those who cross her... and she usually gets away with it. Although whether or not she follows up on her vow depends on the situation. Gaz and Guest have the closest friendship. She proves to be an overly protective bestie, except she denies how much she actually cares about Guest.