Invader Zim is the titular main protagonist (who is considered a villain) of the Invader Zim franchise and the titular main antagonist in the movie Invader Zim: Enter the Florpus. 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 races. 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."
As a villain protagonist, he often has much more screen time than GIR and Dib. He's considered a villain on the morality spectrum (but some may view him as an anti-hero in several cases). The debate about him being a sympathetic villain is ambiguous because he is seen as evil but has been mistreated and ridiculed by his species for being a defective and is oblivious of it. Although his nature is considered bad and he usually only causes chaos and destruction for either himself, his own race, or Earth, he occasionally shows some kindness towards certain characters (such as GIR in "Walk of Doom").
