Zim is, as previously noted, an Irken with megalomaniacal, sadistic, sociopathic, paranoid, and narcissistic personality traits. He usually ends up being a danger to himself and his own people, but he still considers himself a mighty Irken warrior. Though he isn't technically evil, he is truly a human hating xenophobe with full intentions of pleasing his masters and taking over Earth.
Zim was sent into exile by the Almighty Tallests due to Zim accidentaly almost destroying Irk with a massive robot assault walker destroying entire buildings and military vehicles in his path and possibily killing dozens of other Irkens and ruining Operation Impending Doom (OID 1 for short) for not knowing that he was attacking the wrong planet. As punshment Zim was sent into exile and was sent to Food Courtia as punishment for almost toppling the Irken Empire. During the Great Assigning for Operation Impending Doom ll, Zim escaped and made it to Irk where he expected to be given an assigned planet but instead was given a sandwhich and was told to leave. In response to his refusal to leave, Zim was sent to Earth unaware that the Tallests thought it was fake and was given GIR, a SIR unit (Self Information Retrieval Unti) made from garbage and other junk that was defected from the others.