LX-13, also known as Lex Luthor, was an unstable clone from Cadmus Labs in the CW Television series Smallville. 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 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 and 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.