Well, what TV show has no characters? A TV show that is indescribable because of how bad it is. Well, Invader Zim is a great TV show, so of course it has characters. Here are their Biographies:
Invader Zim: Zim is the main character. Determined to Invade a planet and participate in Operation Impending Doom 2, he convinces the Tallest into assigning him a planet. The Tallest act like he is on a secret mission, but he was supposed to get lost in space. Of course, as we all know, he tries to Invade Earth. He is not the Brightest Bulb in the box, but he is very smart, especially compared to all the humans except for Dib. If it weren't for Dib, he would probably have taken over Earth, but Alas, he hasn't. G.I.R. is also a cause of his constant failed attempts.
G.I.R.: Probably the most liked Zim Character, he is also the plucky comic relief. He is dumb and was a 'Top Secret' model made by the Tallest out of scrap parts. I think that since S.I.R. stands for Standard Issue Information Retrieval Unit, G.I.R stands for Garbage Issue Information Retrieval Unit, but I am not positive.
Dib: The only intelligent Human, he is constantly destroying Zim's plans, and can never seem to keep valuable information giving away that Zim is an Alien. Since he seems to believe in a lot of things like Bigfoot and stuff, no one believes him about Zim.
Gaz: Dib's sister. She is really obsessed with Video Games and Beating up Dib. However, Dr. Membrane, her dad, constantly tells her to help Dib. Actually, in a fanfic on www.Fanfiction.net, PEZ: the forbidden story By KnuxZimRyoko15, (this is made up, but it proves an interesting point) it says that Dib's mom dies and Gaz is always mean to Dib because she is jealous that he got to spend more time with their mom.
Miss Bitters: Zim and Dib's teacher. She is very boring, and although she seems and is mean, she is especially gullible. She can only stand so long out in the sun unless she is covered, according to the episode Voting of the Doomed.
Dr. Membrane a.k.a. Mr. Membrane: Dib's dad. He is a scientist and a great inventor. He is kind of weird and seems to believe Dib will eventually work where he works, and that Dib's Paranormal studies are just a phase.
Gnomes: Though not really characters, these are Zim's front yard decorations and his guards. They can move and shoot lasers.
Minimoose: Though he was only in one season one episode, The worst Christmas Ever, He was going to be a main season two character. He was going to float around doing nothing.
Well, though there are more characters, those are the ones that are in just about every episode and actually have names. Yeah, there was Chickenfoot, Keef, Nick, etc., but they didn't appear in many episodes. In fact, Keef is the only one in more than one episode (from my knowledge).