In his first appearance, Brainiac wasn’t even specifically identified as an android. He appeared on Earth as green skinned, humanoid alien, one who tried to shrink Superman’s hometown of Metropolis, along with several other cities on Earth. The original idea for this scheme was that he would regrow the cities to their full size on an unnamed planet on which he ruled, in order to repopulate that world and create a population of enslaved citizens.
It wasn’t long after their first encounter that it was revealed to Superman that Brainiac had also shrunk the Kryptonian city of Kandor before the planet’s destruction, and kept its miniaturized population inside a bottle. This gave Superman a whole new reason to consider Brainiac among his arch enemies, as Brainiac had the only known survivors of his home world (aside from his cousin Kara) trapped inside his bottle city.
After many schemes against Superman, Brainiac’s true origin was revealed – he had been created by the Computer Tyrants of the planet Colu (sometimes called Yod, or Bryak) as a spy to invade other worlds and collect their knowledge. The Computer Tyrants only programmed Brainiac with a Tenth-Level intellect, so as not to surpass their own Twelfth-Level intelligence, and overthrow them. Brainiac was later able to boost his intelligence to level twelve, with the help of fellow Superman enemy Lex Luthor.
As part of his disguise as biological being, Brainiac was given a Coluan boy to raise as his son. This boy, also named Vril Dox, would grow up under the name “Brainiac 2” and eventually overthrow the Computer Tyrants of Colu himself. His descendant, Brainiac 5, would become a member of the 30th Century Legion of Super-Heroes. The original, android Brainiac would also survive into the 30th Century, where he went by the name Pulsar Stargrave, and often came into conflict with the Legion.
After decades of suffering defeat at the hands of Superman, Brainiac built a giant computer generated world under his complete control, with which he hoped to finally defeat Kal-El for good. Instead, he found himself trapped by Superman in the center of this computer planet, and to escape, he had to make the nearby sun go nova, obliterating the entire world, and his own android form.
While his android body was destroyed, his consciousness was able to reincorporate in a new, skeletal metal body, one which appeared much more fearsome than his previous form, that of a green dude in tiny short-shorts. In addition to his new robotic body, Brainiac also built a giant “skull ship” from which to travel the cosmos, which resembled his new head, and in fact was an extension of his own consciousness. In this form, Brainiac terrorized the Man of Steel until the multiverse-shattering CRISIS ON INFINITE EARTHS.
After 1985's CRISIS ON INFINITE EARTHS, the history of the known universe was changed, and Brainiac had an entirely different back story. In this new history, Brainiac started out in life as a biological being, a brilliant scientist on the planet Colu. When he attempted to overthrow the Computer Tyrants of his home world, he was executed. But his powerful consciousness survived in the form of microscopic nanites, and after travelling across the vastness of space, bonded with that of a sideshow carnival mentalist on Earth named Milton Fine, whose stage name was “the Amazing Brainiac.”
Because he needed cranial fluid to survive and maintain his possession of Fine, Brainiac began a murder spree that caught the attention of Superman. Milton Fine, despite being a carnival attraction, actually possessed mental powers, powers that were much more amplified while in the control of Vril Dox.
Eventually, Brainiac was captured by Lex Luthor, but used his mental powers to take control of Lexcorp away from him. Using Luthor’s vast financial resources, he was able to have Lexcorp scientists recreate his original green Coluan body, upgrading from the human form of Milton Fine. His new, advanced mental powers allowed him not only psychic manipulation, but also control of computer systems. Brainiac 2.5, as he referred to himself, was well on his way to becoming a fully artificial lifeform.
It was after the second so-called "Second Crisis," that the real truth behind Brainiac was revealed to Superman, by none other than his cousin Kara, better known as Supergirl. When an android sentry arrived on Earth, one resembling the Bronze Age, Pre-Crisis Brainiac, it was taken out quickly by Superman in battle. It was then that Superman’s newly arrived cousin Supergirl revealed to Kal-El that all the Brainiacs that he has faced over the years are not the real thing, but merely his probes, clones, or nanite controlled bodies, sent throughout the universe. Superman had never met the real Brainiac.
Supergirl also reveals that Brainiac had stolen the city of Kandor from Krypton before its destruction. This leads Superman out into space to find the true Brainiac, in an effort to confront him and free the last of his people. Superman encounters Brainiac capturing a city from a distant world and miniaturizing it, and then destroying that same world. Brainiac believes knowledge is valuable, and after stealing a city from a planet, he destroys that planet in an effort to increase the value of said knowledge.
Kal-El eventually finds himself captured on Brainiac’s vessel, and discovers that he is no match for the true Brainiac’s power. After reading his mind, Brainiac travels to Earth and manages to shrink Metropolis, but is ultimately defeated by Superman. Metropolis is restored to its real size, and the city of Kandor becomes New Krypton.
After 2011's FLASHPOINT event, the continuity of the DC Universe was reimagined once more. In this new reality, Vril Dox was once again one of the greatest scientists on the planet of Yod-Colu, becoming famous due to his discoveries of miniaturization and his advances in force field technology. Thanks to the A.I. he discovered called C.O.M.P.U.T.O., he also managed to discover the Fifth Dimension.
This dimension was ruled by beings called The Multitude, who entered our existence and destroyed hundreds of worlds. In an effort to save his world from the Multitude, Dox used C.O.M.P.U.T.O to take control of the computer systems on Colu, miniaturizing the city his wife and son resided in, and letting the rest of his world be destroyed after he was branded a criminal.
After the destruction of Yod-Colu, Dox constructed an army of robots to travel the universe in an effort to save elements from each world from the Multitude. It is here he earned the name “the Collector of Worlds.” Dox then journeyed through the cosmos, and upon finding a world that is targeted by the Multitude, he would take control of that world’s infrastructure and most advanced computer systems.
He then created an army or robots who took all the aspects of that world he deemed worthy, placed it into one city, and then miniaturized and froze it, letting the rest of the world be destroyed. Each of these planet’s inhabitants refer to Dox by whatever they call their most advanced computer system. On Krypton, he was called Brainiac 1.0, and on Earth, we simply called him “the internet.”
Brainiac eventually arrived on Earth, hoping to miniaturize Metropolis as he had in previous timelines. He also wanted Superman, because he felt his bottle city of Kandor was incomplete without the addition of the last Kryptonian. Superman was able to defeat Brainiac, and even commandeered his ship to serve as his headquarters in the North Pole, his Fortress of Solitude. Brainiac attempted revenge on the Man of Steel many times after this, becoming one of his greatest adversaries. On one attempt, Superman eventually threw Brainiac into a Black Hole. But Dox emerged on the other side….where he met yet another being who went by the name Brainiac.
The Brainiac encountered by the New 52 version was an amalgamation of all the previous Brainiacs from before and after the various multiversal crisis, who mutated into a nearly God like being. This “ultimate Brainiac,” instead of capturing cities from doomed planets, actually captured cities from doomed realities, versions of Earth and other worlds destroyed by the Crisis on Infinite Earths, Zero Hour, and other Multiversal upheavals. These domed cities were placed on a planet outside the known Multiverse, in the Bleed, and watched over by Brainiac’s agent Telos.
After the event known as the Convergence, the domed cities were freed and let go into the Multiverse. The “Ultimate Brainiac” revealed his was now sick and dying from his mutations, and wished to simply go back to being simply Vril Dox again.
After the events of 2016 DC UNIVERSE REBIRTH #1, the pre-FLASHPOINT history of Superman was mostly restored, and it seems the Brainiac he faced in the post-INFINITE CRISIS timeline is the current version of Brainiac once again. Aside from Superman, this Brainiac has also recently fought the Green Lantern Corps.