OKOK so ZIB.. right...
He's like, human-irken... so the PAK is on his head because is that where human brains are.. but Irken brains are in their PAKS so like Dib says in the unreleased episode "ten minutes to DOOM" he says that it IS Zim... so Irkens like database and info can just simply be recoded by Irken control brains (or if you're a defective, absolutely make them gio bonkers) So Zib puts the PAK on his head because he purposefully wants to merge with Zims existence to prove to Urth that the IRKEN threat is real. so he becomes "earths defender" by merging with Zim (and because he's a merger of Zim and Dib, GIR calls this alternate reality version of the human 'Zib') so Zim's data is corrupting Dib, and so he takes over earth trying to defend the urth and kind of accidentally destroys his universe, opening the Zimvoid (a portal into alternate realities) and comes to OUR reality (where the original Zim and Dib are.) But the Zimvoid is actually the shattered remains of Zibs reality. Also Zib is hot.
(ps. i wrote this b4 i read the Zimvoid/Battle void arc)
EDIT: theres also the Zib/Dib virus and Za2r that i would like to talk about.
So into Zib's origin story... (now that i actually read the damn thing...)
Jhonen Vasquez states that Irkens use all caps "because they're annoying." [1] This explains why Zim's name is always written as "ZIM" on the official voice transcripts. It can be assumed that all other Irkens write their names in the same fashion.
However, it should be noted that Zim was trying to read the word [doorbell] with some struggle, implying that he was attempting to learn human writing little by little which he apparently achieved at some point before the episode "Zim Eats Waffles" as he was seen reading the newspaper[fluently/naturally].
In "Battle of the Planets", Irken ships could be seen burning a massive Irken insignia into the Blorch's surface with lasers. This may or may not be standard procedure in order to "mark their territory" and let the rest of the universe know that the planet is now under Irken control, afterwards they give the planet itself a sole purpose.
Emperor Zim’s outfit is purple because, as revealed by Vasquez, this is Zim’s favorite color. Curiously, the color purple represents royalty on Earth, something that probably neither Zim nor his future self would know and on that note, Emperor Zim is wearing a crown, something that Irkens don’t associate with power or royalty (at least not at the moment). Coincidentally, this is the second time Zim did this, as in “Bad, Bad Rubber Piggy” (another time travel related misadventure) he had a drawing of himself on his computer screen with him wearing a crown.
In an interview when someone asked Jhonen Vasquez if Zim would ever conquer Earth, he laughed and said yes which finally happened in this issue.
The following is an account of Vort's history, from their alliance with the Irken Empire to their eventual enslavement, as revealed throughout the span of Invader Zim. - Vortian History, Invader ZIM Wiki.
The Vortian race was originally in alliance with the Irken Empire, and made many powerful weapons for them, such as the MegaDoomer Combat Stealth Mech, Minimoose and presumably the Frontline BattleMech. They also designed most of the Massive, the flagship of the Irken Empire.
However, the Vortians' alliance with the Irken Empire ended on a sour note when Zim created a hideous blob creature on Vort Research Station 9, which devoured Tallest Miyuki, and later Tallest Spork. Because the blob was produced in a Vortian facility, the incidents were unjustly blamed on the Vortians.
HALF THE REASON WHY THE IRKEN EMPIRE BEGAN OPERATION IMPENDING DOOM 1 AND WHY CONVENTIA WAS THE 1ST PLANET THEY EVER CONQUERED IN THE NAME OF THEIR ARMADA WAS BECAUSE THEY WANTED TO ENSLAVE PLANETS THAT HAD SURPLUS AMOUNTS OF EXOTIC JUNK FOOD. IRKEN BIOLOGY IS VERY STRANGE. FOR WHATEVER REASON, IRKENS CAN ONLY EVER EAT JUNK FOOD WHICH IS WHY ZIM'S BODY REACTS SO NEGATIVELY TO HUMAN EARTH FOOD. IT’S ALSO WHY "THE MASSIVE" IS USED AS BOTH A WAR SHIP AND A GIGANTIC TRANSPORT SHIP BECAUSE ITS USED TO LUG LARGE CARGO CONTAINERS FILLED TO THE BRIM WITH DONUTS WHICH ARE THE ALMIGHTY TALLEST FAVORITE JUNK FOOD SNACK. IN FACT, HALF THE REASON WHY IN THE MOVIE WHY THE ARMADA WAS JUST BLOWING UP RANDOM PLANETS INSTEAD OF CONQUERING THEM IS BECAUSE THIS IS SUPPOSE TO EDUCATE THE VIEWER THAT IMPENDING DOOM 2 IS OVER. EVERYONE IN THE ENTIRE GALAXY, INCLUDING THE RESISTY HAS ESSENTIALLY JUST DROPPED ANY AND ALL PRETEXT OF EVERY ATTEMPTING TO OUTGUN THE ARMADA SINCE THEY'VE NOTICED THE ONLY THING CAPABLE OF DESTROYING THE ARMADA ARE DIB, GAZ AND ZIM'S STUPIDITY AND INABILITY TO REALIZE THAT THE ALMIGHTY TALLEST HATE HIM. THE RESISTY BELIEVED THAT WITHOUT THEIR FOOD SUPPLY THE ARMADA WOULD IMMEDIATELY SURRENDER BUT THE PROBLEM WAS MOST CIVILIZATIONS ARE ABSOLUTELY F**KING TERRIFIED OF THE IRKEN ARMADA AND WHO CAN BLAME THEM. THEY'RE USING STOLEN VORT TECHNOLOGY. THE MASSIVE, THE MEGA DOOMERS, EVEN THE SIR BOTS WERE ALL ORIGINALLY BUILT BY ENSLAVED VORT SCIENTISTS. WHEN YOU REALLY TAKE THE TIME TO PROCESS ALL THIS, JHONEN ACTUALLY MADE A VERY COMPLEX LORE DRIVEN SHOW WHEN HE WAS NAMELY JUST TRYING TO MAKE A DARK COMEDY.