Tess Mercer was created for the TV series "Smallville" after Lex Luthor went missing at the end of season seven. She first appeard in the first episode of season eight and was portrayed by the actress Cassidy Freeman.
Storyarc (TV series)
Tess Mercer was born as Lutessa Lena Luthor as the daugther of Lionel Luthor and Pamela Jenkins. After a few years, Lutessa was give up to an orphanage, run by Granny Goodness. Granny trained Lutessa to become the leader of her personal team - the Female Furies. But Lutessa doesn't like the orphanage and tried to escape many times and was always caught by Granny. Later, she removed every single memory of Lutessa and sent her to her new parents who lived in Louisiana. She also was renamed to Tess Mercer. So, she grew up in a swamp house, was abused by her father and taught herself how to read. Tess began to study so she could leave her "parents". At the age of 17 she graduated from Harvard as a marine biologist. Later, Tess and her best friend was kidnapped on a lonely island, where she met Green Arrow for the first time. Oliver saved her life but her best friend was killed by the kidnapper. After she was back in the states, Tess began working for LuthorCorp and a month after that, she was recruited by Amanda Waller to be an Checkmate agent.
After Lex Luthor mysterious disappeared, Tess took control of LuthorCorp with every project, every subsidiaries - she became the CEO and also the publisher of the Daily Planet. Like her every former CEO of LuthorCorp, Tess became obsessed about Superman and his secret. She formed the Injustice League and tried to save the world but she lied to them and was later betrayed by Plastique and Parasite.
Later Tess turned completely good and after Chloe Sullivan went missing, she took over Chloes position as the Watchtower. In the seriesfinale, Tess was killed by her own brother Lex but right before she died, she erased his memories with a toxic mixture in order to keep Clark Kent safe from Lex.
Storyarc (Smallville Season 11)
After Lex met Clark Kent on the streets of Metropolis Tess mysterious appeared behind Lex and said "What's the matter, Lex? You look like you've seen a Ghost." After that, she appeared here and there but it's still unrevealed what happend to her/what she have become.