Invader

"Invaders? These damn invaders have no goddamn choice but to invade any planet, like what Zim has done to Earth... before Tak does."

--Su Ji-Hoon, Operation Impending Doom III

Invaders are the best of the best Irken Elite soldiers. Only an Irken Elite soldier can become an Invader.

Description
They are trained in every way the Irken Empire has to offer and more; experts in planetary infiltration and espionage, Invaders have the capability to conquer their assigned planet single-handedly, after enough information has been gathered, thus eliminating any possible threat before the actual invasion begins. The Invaders are equipped with a small, single-piloted ship and a tiny capsule that can unpack into a fully dynamic base of operations. The ship also provides a device that allows the invader to disguise him/herself, and blend in with their assigned planet's native inhabitants. They are also given a SIR (Standard-issue Information Retrieval) Unit to assist in gathering information about the planet's weaknesses. Once they have their base and disguises finished, they learn about and exploit the planet's weaknesses with help from their SIR Unit; when the planet is sufficiently undermined, the Invaders call in the Irken Armada to finish the job by razing the planet's surface with the Organic Sweep.

How Zim Became an Invader
In an Invader Zim episode "The Frycook What Came from All That Space," it is proved that Zim once truly was an Irken Invader, as when the Control Brain re-encodes him, it shows his status as being an Invader before being changed to Fast Service Food Drone. This is also shown in an Invader Zim unfinished episode "The Trial," when it shows Zim's history. Zim tried to get out of his underground training classes with the help of Skoodge, so it is unknown how he managed to be allowed to take the test. However, what is known is that he passed the test on Devastis and became an Irken Elite, making his way to Invader class.

During Operation Impending Doom, when Zim went on a rampage with a Frontline BattleMech, he apparently blew up all the other Invaders. Presumably, the Invaders present for Operation Impending Doom II were almost an entirely new class.