A clone with Lex's memories, he served as the main antagonist in the Season Ten premiere "Lazarus".
He was portrayed by Mackenzie Gray, who also played Alistair Kreig in the same series, Jax-Ur in the DC Extended Universe, voiced Obadiah Stane in Iron Man: Armored Adventures and Fangtom in LEGO Ninjago: Masters of Spinjitzu.
Originally, LX-13 was created in Cadmus Labs after Lex was badly injured, so Lex could harvest replacement body parts from him. However, due to complications in the cloning process, the clone had a significantly older appearance and proved to be unstable, dangerous, hostile, and was affected by his original self's memories.
He was locked away from his other "brothers".
When Tess Mercer awoke in Cadmus Labs, she encountered a young boy who called himself "Alexander Luthor". He tells her that the other clones were his brothers. Tess heard a banging coming from behind her and just as Alexander shouted not to open it, she does and the older clone of Lex escapes and attacks her.
Tess told Alexander to run, which he does, and she starts to fight the older clone. He recognized her from all his original self's memories, but Tess continues to fight him.
LX-13 got the upper hand and set the lab on fire, saying there can be only one Lex Luthor. Believing that the original Lex was killed in a truck explosion, "Lex" changed into his original self's attire and went to the Kent Farm. Lois Lane was in the barn and delighted to see Clark's new costume with the House of El emblem colorized.
When she heard a creaking sound behind her, thinking it was Clark, she quickly places the lid back on the box and turns around only to be knocked unconscious by Lex's degenerated clone.
LX-13 left Tess alive with a message to give to Clark Kent to meet him in Lawson's Field. The clone took Lois to the same field where his original self saved Clark when he was strung up as a scarecrow during Clark's freshman year. Lex's clone informed her that she is Clark's greatest weakness and attempts to break Clark emotionally by causing her death by setting the field ablaze.
Clark arrives in Lawson's Field and confronts "Lex" who mocks him for the self-righteous attitude he claims Clark has developed and carried with him ever since Clark's very first rescue of Lex's life on Loeb Bridge.
The clone blamed Clark for everything that had happened to him in his original self's life, telling him that it was pride (the greatest of the seven deadly sins) that made Clark become the man he is.
This causes Clark to momentarily lose control of his anger and he begins to choke the life out of his former friend's clone. Clark regains control of his emotions when he notices the blood coming from the clone's nose. "Lex" explains that Clark isn't responsible for his imminent death, but it is only the negative side effects of the accelerated cloning process.
Before his death, "Lex" explains he has set into motion two chains of events to cause the world to hate Clark, which would ultimately destroy him emotionally.
Clark could choose to save the citizens of Metropolis from the plummeting beacon of hope that was set to fall off due to a time bomb or he could choose to save Lois being crucified surrounded by fire in the very same field that he was strung up in as a scarecrow where Lex had saved his life.
"Lex" taunts Clark, saying that he believes he isn't fast enough to save both. As Clark speeds away, the clone dies. "Lex" died as Clark sped off to prevent Lois's death at the crucifix and the deaths of citizens in the rush hour under the falling globe, proving the Lex clone wrong.