During Monty's Galactic Days, Zim was still a scientist for the Irken Empire, and was known as Professor Zim. He was sent by Tallest Miyuki to defeat Monty Uno and his friends in the Outer Limits. He used a robot called GIRbot to battle them, but was ultimately defeated. At the story's end, the new Tallest, Red and Purple, agreed to let Zim be a part of Operation: Impending Doom, but they would later regret this, as Zim ends up banished.
In Operation: GALACSIA, he found out about Numbuh 1, an Earth child, joining the GKND and, to confirm that this was true, he attacked Sector V and forced the info out of them, then revealed his true alien form. He and his robot, GIR, then packed up and left Earth to find an ambush Nigel, hoping to get recognition from the Tallest. He ambushed Nigel and the gang on Planet Secco, then ended up helping them against Invader Tak's Skelbot. He and GIR then went to get supplies on Corneria City, Coruscant, where Zim found out from Star Wolf that the Tallest had added him to their wanted list. After calling them and asking them about this, they lied and said it was a mistake and sent Zim on a false mission back to Earth to kidnap Rachel. While he was getting brutally beat at first, he told Rachel that he knew where Nigel was, making her stop attacking, allowing him to knock her out and kidnap her. After bringing Rachel to the Massive, she and Zim eavesdropped on them saying how they tricked Zim and think he's a failure. It was then that Zim decided to release Rachel and betray the Tallest. During the Irkens' Invasion of Earth, Zim helped Dib save his sister from Sand People, then started fighting off enemies together.
In the future, Zim becomes the new Tallest of Planet Wisp. He married Gaz Membrane and had a son named Giz, who became "Imperator" of the Irken Kids Next Door.
In Operation: NECSUS, Tallest Zim participated in the meeting on Galactic Council, contradicting Tachyon's arguments as he stated the GKND was a reliable force. He then called for King Croacus for support, and the flower king assured it was true.
Zim is a green-skinned Irken with two black antennas and big, bulbous, dark-pink eyes. He wears black gloves, black boots, and a dark-pink dress-like uniform. He wears a mechanical backpack on his back.
Zim is very egotistical and confident in his skills as an invader, but in reality he's terrible. Ironically, he's saved the Earth on several occasions when it was in danger. He once had absolute faith that the Tallest respected and believed in him, unaware of their strong hate for him. After Zim learned the truth, he decided to betray the Tallest and fight with the Kids Next Door to save the Earth.
Zim used to be a scientist of low intellect, so as such, his experiments usually failed, and resulted in the death of Tallest Miyuki's predecessor. Just as well, he is also a failure as a planetary spy and invader, though he has demonstrated skill in being a pilot, and the technology that he uses. When it comes to combat, Zim is not very proficient, as he couldn't lay a single hit on Rachel McKenzie.