GIIIIRRRRR

GIR is happy-go-lucky and extremely hyperactive, and will eat just about anything, regardless of whether it's edible or not. Most of the time he spouts nonsense, but does have a few, short moments of clarity; For instance, in Walk for Your Lives, he actually contemplated whether making the explosion go faster will make things worse ("But if the big 'splody goes fast, won't it get all bad?"), and in Bad, Bad Rubber Piggy, when he questioned the wisdom of sending a machine into the past to destroy Dib.

GIR usually fools around when Zim gives an order, and always goes right back to what he was doing before once he has completed the order (which is only done occasionally) or - more often - when Zim leaves. Zim has described GIR's intelligence as "bad"; indeed, the little robot is often one of the many reasons why Zim's plans frequently fail.
Zim has taken advantage of of GIR's poor intelligence in the past, having once attached GIR to a power amplifier in Plague of Babies to incapacitate Sergeant Shnooky and his underlings with an amplified wave of the little robot's stupidity.

GIR drinking tea and eating cupcakes.

Added by Conker's Bad Fur Day

Unlike Zim, GIR doesn't mind getting involved with typical Earth culture; on several occasions he has even admitted to loving Earth. GIR also has a strong love of Earth snacks, such as tacos, taquitos, candy, chocolate bubblegum, waffles, cupcakes, tuna, biscuits, chicken with mayonnaise, pizza, Suck Monkeys, numerous Poop products, mashed potatoes, muffins and corn. He can eat and enjoy them, even though he is a robot. GIR was shown becoming fat in Halloween Spectacular of Spooky Doom after stealing all of the Halloween candy from trick-or-treaters.

GIR watching his favorite show, The Scary Monkey Show.

Added by Teneery

Despite his sub-par disguises, GIR is actually quite good at blending in with the humans, due to his behavior and intelligence being similar to theirs. In Attack of the Saucer Morons, he even befriended multiple teenage human females. His favorite assignment from Zim is "monitoring Earth broadcasts" (in other words, watching TV); when doing so, he usually watches The Scary Monkey Show, which he has declared to be his "favorite show". His favorite movie is Intestines of War, which he has seen over one hundred times (FBI Warning of Doom). GIR is also very loyal to Zim, despite disobeying him often and mainly being a hindrance to his plans.
There have been times where Dib has "tricked" him into doing things, like putting a camera in Zim's Base in Zim Eats Waffles, and even inviting him into the base during The Frycook What Came from All That Space. This is probably because Dib doesn't seem to bother GIR as much as he does Zim, saying in Planet Jackers that "he seems nice".
GIR doesn't hate or dislike anyone, which contrasts very sharply with Zim, Dib, and especially Gaz.

Duty Mode

GIR's personality changes drastically after an intelligence boost.

Added by Conker's Bad Fur Day

Every so often, GIR will display brief instances of complete functionality. Usually triggered by Zim giving an order, this "Duty Mode" manifests itself in the teal parts of GIR's body and eyes flashing red, and GIR himself snapping to attention, alert and utterly obedient. However, these random bouts never last for long, and after a few seconds, GIR is right back to his gleeful, hyperactive self.
As it turns out, however, this brevity is probably a good thing for Zim: As he learned the hard way when he tried to lock GIR into Duty Mode via a Behavioral Modulator, if GIR was fully lucid all the time, he would quickly realize that his own master was a threat to their mission to conquer Earth, and would quickly eliminate him.