GIR is utterly unpredictable and stupid, which causes him to be almost as hated as Zim. He originally wanted to be disguised as a mongoose, or a mongoose dog. GIR is by far the most innocent, as well as idiotic, SIR Unit ever.
"Duty mode", however, makes GIR exhibit changes in behavior, in which he actually follows Zim's commands. It usually only lasts a few seconds, and is usually triggered when Zim gives GIR an order. When it is triggered, the luminous blue parts of GIR turn red. Zim once tried to lock GIR into to duty mode, but GIR ended up rampaging and attacking Zim, because he realized how stupid Zim really is.
Unlike Zim, GIR loves Earth food, like waffles, suck monkeys, taquitos, tacos, burritos, chicken, mayonnaise, and just about everything else.
GIR was originally given to Zim during the course of Operation: Impending Doom 2. His "brain" was made from random junk, and he is an outdated model 1 SIR unit.