Krang is a character loosely based on the Utrom race from the original comics from Mirage Studios, and an ally of Shredder. In Dimension X he was a powerful dictator/warlord who led brutal campaigns of Rock Soldiers, with fanatically loyal General Traag as his direct subordinate. He also took the completed Technodrome, a powerful mobile battle fortress, and banished Drakus who helped Krang build it to Earth.
One day, an unexplained, bizarre incident occurred that caused Krang to be stripped of his body (which was reptilian, according to "Invasion of the Krangazoids") and reduced to a brain-like form, also resulting in his exile from Dimension X to Earth. After he was banished from his original dimension, along with his operative fortress the Technodrome, they somehow found their way into the hands of Oroku Saki, a.k.a. the Shredder. Krang allied himself with the Shredder, who, along with his robotic Foot Soldiers army, moved into the Technodrome.
Krang pleaded for a new body in exchange for his developed technology and help in conquering the world. Shredder resisted, fearing that his ally could come to rival him; however, he finally submitted once he lost hope for defeating the Ninja Turtles himself and needed Krang's help. He finally constructed the android body from Krang's own blueprints in the Season 1 episode "Shredder & Splintered". Krang returned to his megalomaniac personality, dreaming of bringing an army of Rock soldiers to Earth and ruling it alongside (or maybe without) The Shredder. Indeed, in the season 3 episode "Shredderville", the Turtles have a dream of a parallel world in which they never lived, and Shredder had no problem taking over the world. In this world, Shredder abandoned Krang after his conquest was complete, leaving him with no body and a heavily-damaged Technodrome.
Krang rarely joined in actual combat with the Turtles. Usually the only times he was directly involved with a fight was when the Technodrome was fully up and running. For seven years he organized the plots of the Foot Clan to conquer Earth, although it probably became his objective only after he was exiled on the Earth. He does not share Shredder's obsession with the Turtles and Splinter; while Shredder sees them as mortal enemies, Krang sees them more as annoyances to be disposed of should they interfere with his plans. Shredder's obsession and honor-based impulsiveness ends up ruining Krang's plans more often than not.
Counting from the first meeting between the Turtles and Shredder and Krang, Krang spent seven seasons in the Technodrome, either somewhere on or in Earth or in Dimension X, scheming to power up his battle fortress and take over the Earth.
Eventually the Turtles managed to banish the Technodrome back to Dimension X without Krang and Shredder. At that point they began operating out of an old science building. Due to damage to Krang's android body, he was forced to use his bubble walker while on Earth. Krang and Shredder eventually returned to the Technodrome, only to see it into its final battle in the season 8 episode "Turtle Trek", where the Turtles destroy the engines of the Technodrome and a monster plant pulls it into a vast pit, destroying the trans-dimensional portal and leaving the Technodrome's inhabitants "stranded" in Dimension X, thereby putting a permanent end to Krang's plans. It is implied that Krang and Shredder initially tried to repair the Technodrome, but abandoned it altogether when these efforts failed (although the Technodrome's trans-dimensional portal is shown to be still working in the series finale). Krang's Android body was left in the remains of the Technodrome when it was abandoned. Krang and Shredder spent the next two years wandering Dimension X, looking for a means of rebuilding their power, until he and Shredder were transported to Earth by Lord Dregg.
Dregg used his transporter to abduct Krang and Shredder, presuming that they would help him fight the Turtles. However, both sides double-cross the other, and Dregg ends up draining Krang's intelligence and life force. Shredder escapes and restores Krang, but Dregg captures them again. Finally, the Turtles spoil his plan and transport Shredder and Krang back to Dimension X.
In the series finale, "Divide and Conquer", the Turtles return to the Technodrome to take Krang's android body, which they need to fight Dregg. Krang is nowhere to be seen, but it is assumed that he is still somewhere in Dimension X.
Krang is a major character in the special Turtles Forever in which he, Shredder and the Turtles from the 1987 show end up in the 2003 universe. Although Shredder was able to find his 2003 counterpart, he was unable to find Krang's (even though he do exists in this universe, albeit as a regular, non-evil Utrom). He appears alongside Shredder and is present when he resurrects the Utrom Shredder. At first Krang thinks Ch'rell Shredder isn't that bad, since he physically resembles Krang himself. However, Krang quickly becomes annoyed after Ch'rell takes over the Technodrome and won't allow anyone else to use the Trans-Dimensional Portal. Karai quickly pulls Krang from his android body and orders him to not bother Ch'rell Shredder again.
When Ch'rell completely turns on Krang and the "1987 Shredder", they both join forces with the Turtles once Karai frees them and Splinter. Ch'rell, using the same technology Krang designed for his android body, grows to an immense size. So Krang does the same with his android body, and attempts to fight Ch'rell, but is quickly defeated. This is only the third time Krang takes his android body into battle.
At the end of the story, after Ch'rell is defeated, Shredder and Krang say they are going to use a new Giggle ray Krang invented on the Turtles once they return to their own universe.
2012 TV series
Krang appears in the season 4 episode "Trans-Dimensional Turtles" where it is revealed that he and Kraang Sub-Prime are cousins, and that Sub-Prime banished him to the second dimension. He attempted to redeem himself by using three dimensionizers to destroy the 1987, 2012 and Prime realities. Once both the 1987 and 2012 Turtles thwart his scheme and reveal it to Kraang Sub-Prime, the latter gets angry that his cousin would destroy dimensions The Kraang had been trying to mutate for thousands of years. It was also mentioned that Krang was banished to the second dimension reality because he was incompetent. After giving Krang a thrashing, Kraang Sub-Prime banished him back to the Earth from the 1987 TV series.
Krang is a genius of unchallengeable caliber and knows it, this results in a deep rooted belief that he is superior to all others. This arrogance leads him to see the turtles as minor setbacks unlike Shredder who consider them mortal enemies. A total lack of empathy grants him the luxury of being able to recklessly endanger his "allies" He greatly enjoys the suffering of other living creatures making him a sadist.
He also suffers from low self-esteem (having his very powerful reptilian body ripped apart and turned into a small brain may be the reason for this) as evident by his highly submissive behavior pre android body. He returned to megalomania and arrogance after gaining his new body, using it to become bigger and therefore better.
Powers and abilities
A genius of unrivaled caliber, laser beams, shrink-boomerangs, doomsday devices, goodly doodads, sun draining energy rays, you name it and he makes a better one with missile launchers. With an IQ twenty times that of any human being Krang can concoct Machiavellian schemes and think nine steps ahead of most others. (Unfortunately his arrogance and incompetent helpers can be crippling)
Though physically the frailest of all characters in the series he is capable of slithering at approximately, 3 miles per hour.... And might give you a nasty slap with one of his meaty lumps, or even bite with his tiny fangs. (Those are sharp so watch out) But in his android body it's a whole other story, though he rarely engages in combat with the Turtles he has proven more than a match for them.
Krang used a large, bulky, powerful mechanical body. Krang rested inside a cockpit within the torso. Originally, the suit had a molecular amplification chip installed that allowed both Krang and the robot to change his size to near Godzilla proportions. This chip was subsequently destroyed in the first episode of the 1987 series the suit debuted, though it was apparently repaired in a later episode in which the suit is brought to life by a computer chip from a video game being played by Bebop and Rocksteady, only for the robot's entire body to be destroyed by a powerful explosive developed by an at the time amnesiac Shredder after Michelangelo managed to take control of the machine via his own copy of the same game, though he obviously found a way to somehow repair it in Turtles Forever, as he again uses its ability to increase in size. The Utrom Shredder Ch'rell co-opted this technology and quickly defeated Krang in direct hand to hand combat in a matter of seconds.
According to the episode, "Venice on the Half Shell", Krang possesses an IQ of 968.
It was also in that very same episode that he was able to shake the asteroid they were on just by stomping his robot body's foot on the ground in a fit of rage and damage the Technodrome itself just by banging on it repeatedly.
Even without the microchip, the exosuit was quite powerful. The hands could shape-shift into various weapons (axes, maces, pincers) and even a communication device. In later episodes, however, Krang could be seen changing his arms from a collection of different types he had in his personal quarters aboard the Technodrome. Krang's suit also could "grow" jet wings in place of his arms, which he used to manage a quick escape with Shredder in its first appearance.
Krang also occasionally employed a machine he referred to as a 'walker'. This device was primarily a set of mechanical legs attached to a platform with either glass or some form of transparent but protective covering through which Krang could see, and holes for him to extend his tentacles through so that he could manipulate objects. This particular mode of transport was the version used for the first toy of Krang produced by the toy company Playmates Toys.