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 I) 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 race. There, he became quick rivals with Dib.

Zim's primary desire is to conquer Earth, mostly to prove his superiority and impress his leaders. However, as mentioned by Gaz in "Bloaty's Pizza Hog", "He's so bad at it."

Although he is the protagonist, Zim is morally, and ethically a villain, being an overall fairly unsympathetic character, though he occasionally shows some kindness towards certain characters, (such as GIR in "Walk of Doom"), and is usually only causing chaos and destruction for either himself, his own race, or Earth. As well, almost everyone else he comes into contact with at any time. His unawareness of his destructive actions is most likely the result of him being a "defect", as revealed in the cancelled episode "The Trial".