Invader Slim is an Irken who, as his name suggests, is slim, but that doesn't stop him from exterminating the human race and destroying Earth. Unlike other Irkens, he shows little to no respect for his kind (with the exception of Tak, whom he likes to work with sometimes.), preferring to work alone in most cases, and being immune to human food, making him one of the most powerful Irkens. His SIR Unit is LUR.
Since he despises his own kind, Zim is one of his greatest thorns to his plans. However, they rarely have any intent of killing each other, instead preferring to foil each others plans. Slim thinks of Zim as a failure. However, Slim's ignorant attitude has him mostly, if not always, fail to reach his goal. He was close to killing Zim, but a reprogrammed GIR saves the day by breaking stuff Slim needed to end Zim.
Since the barely interact, Slim and GIR have a neutral relationship. Although GIR once saved Zim from dying in the hands of Slim by breaking stuff Slim needed.
Tak and Slim have a good relationship as they have helped each other on numerous occasions. They both hate Zim and constantly target him. They also have reasons for targeting him as Slim does because of his dislike of most of his kind and Tak does it because Zim ruined her training.
LUR is Slim's SIR Unit and while sometimes he can be annoying, he can also be evil. He is rarely abused by Slim and acts as his best friend. LUR lures Slim's enemies into traps by Slim and also plays with GIR sometimes without Slim being against it, although Zim is bothered by it.