Zim is the main character of the Nickelodeon animated series, Invader Zim. Because of his actions, he is mainly considered a villain with the leading role.
Zim is a member of the alien race, Irken. The objective of the Irken race is worldwide conquest, in which Zim follows. He is psychologically unstable which ranks him defective; Zim is not programmed properly like others. However, Zim’s arrogance says otherwise and Zim proposes he is an aggressor and an almighty soldier. And also due to Zim's arrogance, he was sent on a “covert assignment” to Earth however, Zim's leaders just wanted to rid of him because of his actions (he nearly destroyed his home world). Zim enrols in a nearby elementary school, Skool. It is then Zim's misadventures begin and he becomes quick rivals with Dib, a preteen paranormal researcher.
Zim and his race are heavily based off of the average little green men of the 1950s. Zim is a little (Almighty Tallest Purple describes Zim as the smallest of the race) humanoid-like being with magenta pink eyes (completed with ocular implants) and attire. He sports his own PAK and has intentionally black leggings, shoes and gloves. He has antennas that work as eyebrows and ears that pick up vibration and shift when responding to emotion (much like humans). Zim's most common disguise when leaving his house into the public is his school disguise. The costume involves a wig and contact lenses. When people ask Zim about his appearance, he often refers to it as a skin condition. Zim has megalomaniacal, sadistic, sociopathic and narcissistic tendencies.
Zim could have been considered the first Nicktoon villain to have ever killed anyone had the series not been cancelled, since the unfinished episode The Trial confirms he doomed two previous Irken leaders by accident; and, during a flashback in the episode The Frycook What Came From All That Space, it was revealed that he blew up all the other Invaders at the beginning of Operation Impending Doom I.
Zim may have appeared in Vasquez's comic book Squee! before the start of the series. A poster with featuring an alien that looks Irken and is possibly Zim due to the fact it was wearing a wig like Zim's bouffant wig appears above Todd's bed. The image looks less like Zim, though, since the head is rounded and has mere stick figure hands.
Zim has been seen in a commercial on Nickelodeon holding a sandwich to which he has bad allergic reactions to (unless it is an Irken sandwich). In it he was knocked off a wire on which he was sitting on and thrown into a light, making it explode. This commercial was a crossover of other series on Nickelodeon. After the cancellation of Invader ZIM, Zim was replaced with Jenny from My Life as a Teenage Robot. Jenny wasn't knocked into the light.
Zim is known to smell things and scents even though he does not have a nose, or his nostrils are so small that they're not visible, as evidenced in the episode Germs, when he is seen sniffing the scent saying it was the "stink of clean".
In Jhonen Vasquez's "Meanwhile..." comic "Jhonen Vasquez vs. the Devil", There are aliens flying a ship similar to Zim's Voot Cruiser, though the aliens inside the ship look nothing like Irkens.
In one episode of Nickelodeon's Fanboy and Chum Chum, entitled "Dollar Day", Zim can be seen on a comic cover in the background. His skin is faintly green and he is wearing his disguise, however, so one must be careful when looking for him.
At some point in the series, Zim had learned to read human writing, as he is seen reading the newspaper, and understanding what it says, in Zim Eats Waffles.