dib is an irken who is rather tall, making him seem like the excellent candidate for an invader. however, going against orders and basic PAK code, dib develops an insatiable interest in finding and exploring the mysterious planet known as earth, which he believes to be full of fascinating cryptids. his crazy ideas and weird obsession drives the almighty tallests to assign him to the impossible task of "invading" this elusive planet with the hopes that getting rid of dib will keep his abnormalities from causing embarrassment for the other tallers.
zim is a human who was made to be a clone of professor membrane, but an accident in the lab exposed him to chemicals that made his skin green and caused him to be very short, in addition to giving him an enlarged ego too big for his body to hold. while he has an advanced brain and processing capabilities, he seems to only be able to put his mind towards coming up with insane plans and destructive experiments.
even professor membrane is scared of zim. since his conception, zim has been nothing but trouble. he's almost destroyed membrane's labs more times than the professor can count, almost nuked the whole city, set a huge fire without anything flammable nearby, and has tried to trick his classmates into being experimental subjects enough times to get him nearly expelled.
zim still refers to other people as "humans" or "pigs" because he thinks himself above them, despite being looked down on by his classmates.
meanwhile, dib is a kind of smash up between his pilot and movie selves. he has a cheerful, energetic, and easily curious personality that comes across as bubbly or cute on the surface, but he also has a freaky, obsessive side that borders on sadistic and even yandere. this especially comes out more when he meets zim.
zim's only friend is a small robot-dog he made as a kid, who he named gir, being the only sound the puppy could make at the time. but thanks to exposure to many different chemicals, gir has developed the ability to talk and walk on two legs, and he's somehow crazier than his own master.
dib's only companion on the journey to earth is a bossy SIR unit named gaz, who definitely doesn't enjoy dib's incessant conspiracy theorist rantings and doesn't want to do much besides gaming and eating. however, dib seems fond of her and she is surprisingly tolerant of him when she's in a good disposition.
as luck would just have it, dib crash-lands in zim's neighborhood, setting up his base quickly and gleaning enough information to know he will have to disguise himself as a student at their "skool."
zim is the only one who recognizes dib as an alien from the start, trying to get the other students to notice dib's green skin, the PAK on the back of his head, and his strangely unmoving, hard-as-stone hair, but this only seems to make the others suspicious of zim instead.
zim and dib reach a mutual conclusion to perpetuate rumors that they are somehow related (half-cousins six times removed) in order to get by unnoticed, which seems to work for now (mostly because no one cares).
the two develop an immediate and very intense rivalry. zim wants to expose dib as an alien for his own means, while dib becomes creepily obsessed with zim's mutations and extreme personality, considering him to be a cryptid among his own species.
throughout his childhood, zim consistently kept coming up with schemes to get famous, including being an olympian or even a movie actor, but as he failed all of those so spectacularly, he has been coping by believing he is meant for something much greater than any of that. as such, zim could really care less about saving the world, honestly. the only reason he wants to expose dib as an alien is because he wants to make the world's greatest discovery ever and be immortalized as the pinnacle of genius and scientific achievement.
dib also could care less about actually conquering earth (though he gives it his best since zim seems to respond quicker when dib throws a bone for him to chase), and instead fantasizes about dissecting or even keeping zim in a cage. however, despite being given a number of chances to do so, dib always hesitates, concluding it would be better to observe zim in a more natural environment (that's surely the only reason why).
they both use cameras to spy on each other's home lives, but never seem to be aware the other one is looking in on them.
the two spend years locked in their rivalry, even after the almighty tallests have travelled further into space than dib can possibly contact them in. and several times the two have had to work together to prevent dib's identity from being revealed (zim claiming only he should be allowed to expose dib) or to save the earth from their own accidents or from tak. each time they end up learning new things about each other (such as dib enjoying human food despite its poisoning effects on him, or zim hating water but loving sweets) and sometimes go as far as to compliment or even praise each other's newest creations and experiments, which always sends them into embarrassed blushing and insult-trading battles.
dib takes a strong dislike towards zim's bullies, going as far to target them specifically. where zim could care less about being bullied, dib very much doesn't like anyone else trying to get close to his "subject" and hates the human need to belittle people who are weird or different, having experienced that in his home world himself.
as they get older, eventually heading into college, their misadventures start to cool down a bit, especially after they start up a roller coaster on-and-off pseudo-romantic relationship (both of them denying its existence and claiming it's only a ruse to gather information on each other).
but on occasions, they will go out of their way to set up elaborate traps, schemes, or battles for each other... suspiciously on days that mark anniversaries of their past greatest battles and the start of their non-dating.
they are both extremely possessive and co-dependent, meaning even when they "break up" they have never even considered being with anyone else but each other. however, zim seems to take great pleasure in riling up dib's immensely possessive side, going as far as pretending to date keef to instigate the other boy. they both quickly learned to never attempt something like that again after dib created a retractor beam to try to crash the moon into keef's house in a fit of jealousy.
years into the future, when they both have long forgotten their original missions, dib and zim find themselves somehow married with a small irken-human child that gir and gaz take turns babysitting while the two work as academic rivals who consistently argue and try to disprove each other's theories, much to the annoyance of other professors who don't even see dib and zim as "real" scientists.
but they are quick to get recognition when they are forced to work on a unified theory that shows the irken empire has suddenly had a change of heart and is closing in on earth to destroy it. being the only two capable of fending off the empire, they become global heroes after working together to save the earth. but of course that only leads to more competition as they argue over who did the most work and deserves the most credit.
as their half-breed child grows up, dib tries to teach the kid irken customs and technology while zim becomes surprisingly maternal and overly protective, finding a strange new desire to give the love and care he never had to their child. it doesn't help that the kid looks so much like his stupid dad, which gives zim the weirdest warm feeling.
and after their child has grown up to become a famous comic writer, known for their "fantastical" series about growing up as the kid of an alien and the world's biggest narcissist, zim and dib finally settle down fully and reflect on their lives together and how much they've changed. dib being more sentimental talks about how lucky he was to find earth and to meet zim, while zim just huffs and says he's equally lucky dib never deigned to dissect him.
but the two blush in silence as they watch a night sky full of stars, before they finally turn to each other and say the three words they've never said till now... "i love you."
