Despite being blatantly disobedient and often a hindrance to the mission,

Zim often has to put up with his SIR Unit, GIR. Typical of Zim's character, he declares his dislike of gir openly and often, knowing that not only will gir disregard everything he says, but that he probably won't understand him anyway. Zim's dislike of gir most likely originates from Zim's egotistical nature, and GIR's erratic behavior and lack of obedience.

However, at times, it is shown that Zim thinks of gir like a little brother. GIR, despite being defective, is devoted and loyal to Zim, shown mostly through his rare moments of clarity, or in some insane fashion such as wildly attacking him with hugs. Zim is often shown trying to steer gir in the right direction, even if he states

that it is for the good of the mission. gir simply tries to get Zim to have fun every once in a while, despite the fact that Zim refuses to do anything for his own benefit.
gir is often able to bend Zim to his will. Despite often being harsh and stubborn, Zim is known to allow gir to get away with some of his antics. In Bad, Bad Rubber Piggy, Zim lets a teary-eyed gir play amongst piles of their rubber pigs after giving him a firm "No" moments before. Additionally, in Walk of Doom, Zim forgives gir surprisingly fast after getting them stranded in the city when he is faced with the weeping robot.
However,

their bonds of loyalty to each other are not always strong. In Nubs of Doom, Zim would have tried to rid himself of his SIR Unit, replacing him with a new sidekick named Minimoose. Although this plan does not go smoothly, Zim does tell gir that he is "horrible". GIR, however, does not absorb the comment. In Roboparents Gone Wild, gir leaves behind a helpless Zim in favor of fun and destruction. Despite a pleading and rather desperate look to his robot, gir fails to save his master from being ousted.
Their relationship can be seen in almost every episode. However, a few have critical points in their development.
Walk of Doom
After realising that gir left his guidance chip back at the base, Zim snaps at him. GIR, who just ate his cupcake, begins to cry, and Zim sighs, and tells gir "I see that you understand your mistake GIR, and me being mad at you will get us no closer to home." However, gir was really upset because he missed his cupcake. This is also one of the only times that Zim shows remorse for his treatment of GIR.
Battle of the Planets
gir sleeps on Zim's head twice while they are heading to Mars.
Germs
An example of GIR's total disregard for Zim's reprimanding is when Zim goes on a germophobic rampage, and GIR, oblivious, hugs him while smothered in germs.
Invasion of the Idiot Dog Brain
When Zim first realizes that gir is unresponsive, he sounds mildly concerned. However, the mess that gir makes of the

ensuing situation undoes any concern Zim may have had for his robot.
Dia das bruxas Spectacular of Spooky Doom
When Zim escapes the Nightmare version of Skool, he expresses concern for GIR's safety, saying "I only hope poor gir is all right por himself".
Bad, Bad Rubber Piggy
gir hides dozens of piggies in his head, in hopes that Zim won't find them there and he would get to play with them, and not have them tossed into a Time-Space Object Replacement Device. Zim only has to give him a suspicious look to reveal that they are there, but an equally sad look from gir makes Zim allow gir to play with them before they send it into the past.
gir Goes Crazy and Stuff
When

Zim decides to fix his behavior glitch, gir shows his true colors. He reveals that he cannot stand Zim and that he feels his a horrible leader and/or master. When asked if he remembered his time of servitude in the past he responded with, "Yes. I didnt like it." This shows that serving Zim is mais of a hassle then a pleasure to GIR. The creator of the show, Jhonen Vasquez, is quick to note in the commentary for "Best Friends" that gir just wants to die and hates life. This may back up the reason for such hatred and strong angry feelings toward Zim in "GIR goes Crazy and Stuff."(edited) However, this could be because gir is mais mature in this episode.
Planet Jackers
Zim might trust gir for his own safety. When they stop por Dib's house to get his telescope, Zim gives gir a worried glance. gir returns the gaze and smiles (although, when Zim is about to ring Dib's doorbell, gir pretends to crush him with his finger since Zim is in a distant background).