After single-handedly ruining Operation Impending Doom One, Zim was banished to Foodcourtia, the food court planet. Later, he escaped, and begged the Tallest to give him a mission in Impending Doom Two.

Seeing an opportunity to get rid of the short annoyance, the Tallest gave him a ‘secret mission’ and sent him flying off into space. They thought they had heard the last of him, but Zim did find a planet.
Earth.
Quote: I AM ZIM!!

Dib: Dib is a human almost obsessed with the occult and paranormal. Everyone thinks he is crazy. No one at his Skool likes him and he has no friends. When Dib intercepted a transmission from a far away planet, he started to watch for an alien coming to Earth. He didn’t have to wait long, because barely six months later, a boy showed up in his class.
The boy had green skin, no ears, no nose, and no eyebrows. He tried to pass this off as a ‘skin condition.’ Dib was not fooled, and immediately announced to his class that the new boy (Zim) was an alien.
They laughed at him.
Now, Zim and Dib are in a constant bitter struggle. Zim trying to destroy the planet, Dib trying to destroy it. It sometimes seems like a lost cause, since the earth seems to hate it’s sole protector. But Dib battles on.
Zim and Dib are mostly about even. Zim has an excessive amount of technology at his fingertips, but Dib is way smarter, and has more common sense. The battles normally end in a tie, (don’t tell that to Zim, he usually claims some victory or another.)
Plus everyone says his head is big, even though it really isn’t. FUNFACT: America hates Dib.
Quote: I’m not gonna let him get away with his… things he do!

GIR: GIR is an insane robot the Tallest gave to Zim, instead of a SIR. (Standard Information Retrieval unit.) No one knows what the G stands for. GIR is disguised as a dog. He is just… really really insane. Totally random. He has no idea what is going on, ever. He just does whatever he wants to. Zim keeps him around because at random times he decides to obey for three seconds. You can tell he’s obeying because he turns red, like the SIRs. He is usually really destructive but sometimes he’s less destructive. There’s three pics of him. One blue (normal, insane) Red, (paying attention for once) dog (disguised) and then a picture of a normal SIR.
GIR has a lot of cool stuff in his head, like lasers, but he never uses them. He prefers to keep squeak animals in his head. He likes all kind s of food, and DESTROYING!
FUNFACT: Jhonen, being the creepy guy that he is, hated teh fact that everyone loves GIR. So, at certain points in the show, for one frame, transparently, he would insert a 'bloody GIR.' The REALLY scary part? PEOPLE WENT AND LOOKED THROUGH EPISODES FRAME BY FRAME AND FOUND THEM,
Some people have WAY too much time.
Quote: I wanna chair made of cheese and a table made of cheese, and a…

WARNING: Gaz is not happy. Don't act that way while reading her bio.

Gaz: Gaz is Dib’s scary sister. She likes Pizza, Gameslave, piggies, Gameslave II, soda, toast, video games, going to Bloaties, computer games, drawing, trying to destroy all life on Earth and beating up people. Gaz is a year younger than Dib but a thousand times scarier. She knows Zim is an alien trying to destroy Earth, but she doesn’t care. Mostly, she says, because he’s so BAD at it.

Quote: From this day, until the end of the day, Dib you will not know the meaning of peace, for I will rain misery down upon your pizza stealing heart!"

Professor Membrane: The Professor, inventor of the perpetual energy generator and SUPERTOAST, is also father to Gaz and Dib. He is not around much. Mostly he is in his lab working on some invention or another. He is always trying to get his poor insane son to take up REAL SCIENCE. Dib is having none of it.
The Professor also hates Santa because Santa gave him socks one year for Christmas. Whenever Dib needs some high-tech piece of equipment, the Professor is there.
Except for when he forgets that Dib exists.
Quote: SON! There had not be any WALKING DEAD up there! (to self) Always with the DEAD, that boy.

The Tallest: Irken hierarchy is based on height. The taller the better. Red and Purple are EXACTLY the same height. They rule together. They are both pretty stupid. Red is slightly less stupid than Purple. I don’t think they have any real power. They just go around in the Massive, (Armada’s Command Ship) and eat snax and make speeches and stuff. Plus they blow stuff up.