Tnek Kralc

Superman starts his current career the same year as Batman. He's just out if school and starting his job at the planet. He's like Bruce and most of this gen is ready for adulthood out of college. He grew up on a farm after all. So the crux of Donner 1 happens in his first year with Luthor while he's trying to balance a job and whatnot. Lex created Bizarro as a first failed attempt to create his own Superman, a recurring plight. Perhaps he tries to bring another reality's Superman but selects the Bizarro world, or replicates the DNA from such. Konnor is a continuation of this. When LoD forms a year later, after eschewing Bizarro, who although powerful, is hard to wrangle, more or less benign, and brings shame, so Luthor has to try to appease the poor child to help him in the Legion. He acts like he's not exploiting him but he is. He kind of feels bad about it because ultimately he's still human. He's fundamentally "retarded," so to speak
"The first known encounter between Bizarro World and the "surface world" occured when Lex Luthor performed an early trans-realital experiment, in which he attempted to access a subservient Superman from another reality that he could use as his pawn. However, being the first on the bleeding edge of inter-reality travel, with all of Luthor's genius not even he could even realize that his acceleration of the strings was off by some orders of magnitude. When Luthor performed the experiment, he inadvertently pulled the Bizarro World Superman, Tnek Kralc, through to his world (where is Superman story). Almost instantly rejecting Bizarro Superman as a failure, Bizarro would escape confinement and unintentionally cause destruction in Metropolis. The Superman of the "Rightside Up" world would arrive to stop him, finding that their power sets were equal and opposite-- Superman had frost breath and heat vision, and Bizarro had heat breath and frost vision. After their initial fight, Superman went to Lex Luthor to attempt to help Bizarro return to his world. Instead, Luthor saw this as an opportunity to have a test subject to learn Superman's powers first hand so he could find methods of countering them. LOD.
Later, in an act of kindness, Jason Todd would break into LexCorp and attempt to return Bizarro to his homeworld, only for the plan to backfire-- instead of sending Bizarro Superman back, the rest of the Bizarro Justice League arrived through to the Rightside Up."