Zim is based on the stereotypical "little green men from space". He has green skin, is three feet tall, and posesses a pair of antennae which appear to function like ears. He wears a magenta unifrom which is eitehr striped or segmented, a peach collar, peach shoulderpieces and sleeves, black gloves, and black pants and boots.
Zim's PAK is essentially a secondary brain; it contains all his memories, his personality, and is also a useful tool, as it contains 4 robotic spider legs that can also be used as laser weapons, a teleporter (used once by Zim as an organ harvester in an attempt to appear more human), a communicator, and other sorts of tools and devices. Like all Irkens, Zim's PAK sustains his life, and as such, if his PAK is removed from him, he will die in exactly ten minutes. PAK's can be reprogrammed by the true leaders of the Irken Empire, the Control brains, to fit an Irken according to his/her needs (or as punishment). Zim, being a "defective" Irken, can resist his programming (He himself is actually a fast food drone).
Zim is, as stated above, and egotisitical Irken with emotional instabilities. He is bent on the destruction of the human race and/or Earth, but he is extremely incompetent, and unable to do so. He has sociopathic, paranoid, narcissistic, sadistic, and ocassionally insane tendencies. Overall, he is usually a danger to himself and others around him.