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I am Krypton.
~ Brainiac, to Superman, "Stolen Memories" (STAS)
Brainiac is the main antagonist of season 1 and 2 Superman: The Animated Series and one of the main antagonists (alongside Lex Luthor) of season 2 Justice League Unlimited. He is the central computer system of Krypton and one of Superman's greatest enemies.
Brainiac becomes aware of Krypton's impending destruction, but finds the planet's destruction of little importance in contrast to the importance of all of Krypton's data. He thus abandons Krypton when the planet is about to be destroyed, and creates a new goal for himself: to collect all the data of a planet and then destroy the planet. He views this as forever preserving the completeness and uniqueness of this information. He also believes that the fewer beings who have the information, the more precious it becomes.
Years after Krypton's destruction, Brainiac kills a group of aliens and pilots their ship to Earth. Intrigued that Kal-El (otherwise known as Superman) exists, he seeks to bond his DNA with that of Kal-El's to continue his quest for data. Superman destroys the Brainiac drone, but Brainiac continues to be a threat as he downloads his program into LexCorp's computers.
Brainiac orders Lex Luthor to rebuild his body at one point, and then injures Luthor with a laser beam. Superman defeats Brainiac again, however.
Eventually, Brainiac seemingly attacks Apokolips, but Darkseid secretly makes a deal with Brainiac to lure in Superman. Darkseid hacks into Brainiac's system and forces Brainiac to self-destruct, seemingly destroying Brainiac for good.
It is later revealed that Brainiac had spared Luthor only to ensure his survival inside of Luthor. Brainiac planned to implant his mind into that of an Amazo droid, but was stopped by the Justice League. Brainiac flees from the Justice League using Luthor's body. Luthor questions Brainiac's motivations enough to interest Brainiac, and the two merge together with Dark Heart nanotechnology to become an ultimate being. Brainiac desires Luthor's creativity and Luthor desires the power and knowledge that Brainiac has but otherwise has no idea what to do with.
Brainiac/Luthor decide to create a machine that will absorb data of planets in a single stroke, but they are stopped by the Flash.
Brainiac is seemingly gone from existence, but still exists within the mind of Luthor. Luthor discovers that Gorilla Grodd has the last existing piece of Brainiac. Luthor desperately tries multiple manners of activating the one scrap left of Brainiac but it does not work.
Eventually, Luthor goes to the location of Brainiac where he was destroyed and attempts to resurrect Brainiac using magic, but surprisingly resurrects Darkseid instead. The psychic link is shattered from Luthor, thus ending Brainiac forever. Luthor swears revenge on Darkseid, who now seems to have some of Brainiac's power as well.
Some fragment or contingency aspect of Brainiac makes an attempt through time travel to kill young Clark Kent in Smallville, but is opposed by the Legion of Super-Heroes, who are still hard-pressed to protect Clark and his human parents from the AI's rampage. Enraged by his family being endangered, young Clark exceeds his current potential and pummels the attacker. Brainiac's retreat is an erratic one, and he ends up flying into the sun, where he is clearly shown to be destroyed.
In a later Justice League Unlimited episode, Supergirl moves to the 30th Century for various reasons, one of which is to be with a hero called Brainiac 5, an organic whose connections to the AI are not made clear in this version.
A variant on this Brainiac, also voiced by Corey Burton, and having an aspect and motivations nearly identical to the DCAU version, appears on the Legion Of Super Heroes animated series. At first, he is merely a relic, a prior incarnation of the heroic Brainiac 5.0 (the original is called 1.0 in this) that 5.0 calls upon to solve a problem his current program cannot handle. He ultimately becomes the final Big Bad of Season 2, corrupting his 'descendant' Brainy into deep villainy, till Superman prevails upon his old friend to abandon that path. Seemingly destroyed, the remnants of 1.0 are shown at the tail-end of the final episode once more rebuilding themselves. While many things are the same between the two original Brainiacs, The Legion Of Super Heroes animated series is in a different continuity from the DCAU.