Lena was Lex's little sister, who lived with their parents. She inherited telepathy when she was zapped by a Space-Brain in her brother's lab. When Lex became a criminal, she changed her name along with her entire family to "Thorul" out of shame (Though she was not told the reason for the change). Her parents were then killed in an auto accident, orphaning the little girl and leaving her unaware that she is the sister of Lex, the famous criminal. Lex, however, remembered her and intentionally remained unknown to her, as he still cared for her and didn't want to break her heart or get her involved in his criminal activities. ("The Girl with the X-Ray Mind!")
She had tried to get a job with the F.B.I, due to her powers, but they turned her down because of her mysterious past. She first met Linda Lee at a Midvale High science club meeting, where a scientist was testing the possibility of extra-sensory perception. Linda, secretly Supergirl, observed that Lena got every question right on an ESP test. The two girls began talking, and Linda's adoptive family gave Lena a ride home, during which her powers were further in evidence. Later, as Supergirl, Linda investigated the reason for the F.B.I's rejection of Lena, and discovered the connection to Lex Luthor. Meanwhile, Lena, having been rejected by law enforcement, took a job with criminals, leading Lex to worry she was following in his footsteps and to contact Supergirl for help. But Lena had only joined the gang in order to bring them all to justice. Supergirl, discovering this, spoke to the F.B.I about re-opening her case. ("The Girl with the X-Ray Mind!")
While her application was being reviewed, Lena maintained a job at the Midvale Library, keeping up her friendship with Linda. At a costume party, due to a mishap, Lena was mistaken for Supergirl (as she was wearing a Supergirl costume) which lead Linda's boyfriend, Dick Malverine, to believe that Lena was Supergirl's secret identity. He then began to court her, seemingly forgetting about his relationship with Linda. ("The Girl who was Supergirl's Double!")
Supergirl: Cosmic Adventures in the Eighth Grade
She was nice, and a good friend; however, she was sensitive and easily angered, using her scientific prowess to do things such as removing everyone's meteor-given superpowers.
30th Century Version
A woman named Lena Luthor appeared at the head of Luthorcorp in the 30th century. Her family was no longer feuding with Superman's by this era, and she helped end the fight against Muto. (Superman 2999)
In Other Media
In the television series Smallville, the character of Tess Mercer was introduced in Season 7, who initially was a close follower of Lex Luthor and was devoted to him. After discovering his deception towards her, she turned against him.
It was revealed in season 10 that Tess was raised in an orphanage by Granny Goodness, having been left there by her biological father, who was revealed to be Lionel Luthor. Tess soon recovered her birth certificate, which revealed her true name as Lutessa Lena Luthor.