The PAK is a backpack-like device that is grafted to the spine of every Irken upon their hatching. Serving as a secondary brain, the PAK contains all of an Irken's personality and statistics. It's never revealed what PAK stands for, or if it stands for anything at all other than a variation on "pack".
The device contains all of the Irken's knowledge, effectively serving as a auxiliary brain, and it holds a great array of weapons and tools. It also contains the host's personality, charging cell, atmospheric processor, and a handy auto-reset that will "reactivate" an Irken if they suffer a fatal injury ("Plague of Babies"). The Irken host's job is encoded into the PAK's programming, and can be changed instantly by a Control Brain (which can also delete all data in the PAK; such is known as deactivation).
The PAK demonstrates to perfection the cybernetic development, and reliance, that Irkens possess. Unable to live without them, and essentially composed by them, the Irken and their PAK are essentially one and the same thing. They also demonstrate the domination of Irken society by the Empire's Control Brains, which have complete jurisdiction over the status of all PAKs, able to rewrite them, encode them, and where necessary, to delete them. It's unknown when or how Irkens became so reliant upon their PAKs for basic life support, or in what manner the PAK and other cybernetics differ modern Irkens from their pre-cybernetic ancestors.
Irkens can only survive 10 minutes without their PAK (explained fully in "Ten Minutes to Doom", an unfinished episode); once it has been removed, a 'Lifeclock' appears in their mind's eye, counting down how much longer their natural body can last. The longer the Irken goes without it, the more incoherent and uncoordinated they become, and it seems to take a lot of willpower for them to muster up any strength or logic in their PAK-less state.
It's been revealed a few times that the PAK is sentient in a way, as it can act outside of the will of its host. In deleted footage from the episode "Abducted", Zim falls through fire and is momentarily unconscious as he takes a nasty fall. His PAK legs activate, catching onto a ledge and carrying its limp owner to safety.
And given the chance, if a PAK is separated from its host, it will soon begin to seek out another, though it will at first attempt to reconnect with its original Irken owner. If this fails, it will attach to the first being available, and will slowly begin to replace the new host's personality with that of the Irken's. However, if the host is not Irken, then they, too, will perish after ten minutes, but in this case after ten minutes with the PAK on.
The PAK also has an "emergency attachment protocol" and is capable of connecting with other devices as was the case in Issue 25 in which cables came out of Zim's PAK and connected to Fitzoo-Menga's chair, temporarily giving Zim control of the hovering chair, dubbing himself "ChairZim."
As well as containing the Irken's knowledge, the PAK also contains their memories, so Irkens would be unable to get amnesia if they receive head injury.
PAKs have been seen to contain the following tools:
- Four mechanical, spider-like legs which can be used as:
- Audio and/or video communicator (Zim usually uses one to talk to GIR in distant places). This is the first function of the PAK used in the series, seen in "The Nightmare Begins".
- A portable container for any variety of personals.
- The Tallest seem to have levitation systems that can be used indefinitely.
- A jetpack (as seen in the episode "Planet Jackers")
- An invisible bubble helmet used for space travel. ("Battle of the Planets", "Planet Jackers")
- A binocular helmet that has Night Vision Goggles built in. ("Plague of Babies", "Planet Jackers")
- A small robotic arm extending from inside the PAK.
- An organ-harvesting device
- A set of ray guns attached to controllable appendages (seen in the comics)