Invader Zim is scattered with cameos, mainly featuring the crew behind the making of the show. Early episodes like "Germs" and "The Wettening" featured cameo appearances by the creator Jhonen Vasquez and the director Steve Ressel. However, cameos became less and less frequent when Vasquez decided that they were too distracting.
Many other cast members made small appearances in the show at one point or another, such as Rosearik Rikki Simons in "Attack of the Saucer Morons". Others like Roman Dirge also made a debut. However, fictional characters also show up; for example Vasquez's comic character Johnny from Johnny the Homicidal Maniac appears in "Halloween Spectacular of Spooky Doom".
Characters who look like humorous versions of other cartoons also appear, such as what appears to be a Jimmy Neutron parody that was supposed to happen in the cancelled episode "The Trial", according to the concept sketch (also designating him as "Horrible Cartoon Kid"). There is also a child in one of the TV shows GIR is watching that looks similar to Timmy from The Fairly OddParents! in "Parent Teacher Night".
Aaron Alexovich (Alexi Aaronovich)
In the concept art, the name of the character is Alexi Aaronovich.
Frank Conniff is a writer for Invader Zim and perhaps best known for his role as TV's Frank on Mystery Science Theater 3000. He is referred to by name in "Attack of the Saucer Morons", having foretold the prophecy of Zim's coming. In "Hamstergeddon", he was crushed by Ultra-Peepi due to being distracted by his cuteness. He later appears drinking a cup of coffee in "Door to Door".
Courtney Lilly is a writer for the Invader Zim series. He appeared in "Invasion of the Idiot Dog Brain", trying to play what looks like fetch with a dog until GIR shook off dirt on him. Later, in "Battle of the Planets" he was almost crushed by Mars while having his Poop Cola. Lilly's last appearance was in "The Most Horrible X-Mas Ever".
Presumably she also was part of the crowds that later tackled Dib at Zim's encouragement.
Eric Trueheart appears in "The Girl Who Cried Gnome", advising Dib not to eat the "muffins" that Zim's Robomom made.
Jay Bondy is the color director for the Invader Zim series. He was splashed with mud by GIR in "Invasion of the Idiot Dog Brain", along with Courtney Lilly. He later got distracted by the Megadoomer and tripped on his doorstep while walking into his house in "Megadoomer".
Jhonen is the creator and head writer of the Invader Zim series. He was the "too ugly" disguise in "The Nightmare Begins" for Zim as he was trying to find a correct disguise for him. In "Germs", Zim examines his and Steve's burgers in MacMeaties once finding out it is germ-free. In the background, Jhonen and Steve can be seen eating together and seem to be discussing the script for "The Nightmare Begins", as the script can be seen in the background.
He also makes another cameo appearance in "Attack of the Saucer Morons" as one of the saucer morons. Also, he can be seen in what seems to be an aquarium in "The Wettening" reading a comic. When he attempts to drink his water, a piranha jumps into his cup of water and then chokes on it.
Rob Hummel is one of the writers for Invader Zim. He was seen on the milk carton in "The Nightmare Begins" stating "Rob Hummel is Meat". More prominently he appeared in "FBI Warning of Doom" as the enraged Video Store Manager.
Roman is one of the writer/character designers for Invader Zim. He attended as one of the parents in the background of "Parent Teacher Night", made a cameo in "Walk of Doom" and appeared as one of the saucer morons in "Attack of the Saucer Morons".
Roman Dirge has two different cameo designs, this second design being visible later in the series, such as "Hamstergeddon". More notably this version can be seen on the back of the Invader Zim Volume 2: Progressive Stupidity DVD stating: "Now with 100% more Dirge".
Rosearik "Rikki" Simons
Steve is the director of all the Invader Zim episodes, except for the Pilot. He first appeared as one of the disguises offered to Zim that he deemed "too stinky" in "The Nightmare Begins". He can be seen with Jhonen in MacMeaties in "Germs" while Zim examines their burgers after he realises it is germ-free. He and Jhonen also seem to be discussing the script for "The Nightmare Begins", as the script is seen in the background with them.
In "The Wettening", he bought a hot dog and got washed away in the water after all of the water was drained. He also never appeared in "Career Day", on which he was supposed to be paired with one of the kids.
He is also seen to be one of the cast members in "FBI Warning of Doom"'s movie Intestines of War. A parody of his name, "Ress Stevel", can also be seen in the credits of the movie.
Tavisha is Rikki's wife. She first appeared in airdate order in "The Most Horrible X-Mas Ever" found in the crowd while Zim was impersonating Santa Claus (next to her husband), and first appeared by production order in "Mortos der Soulstealer", where she can be seen buying clothes at a rubber clothes store. She would have appeared more often had the series continued.