Tess Mercer (Cassidy Freeman) was a marine biologist from a broken home who, upon joining LuthorCorp, quickly proved her ambition and capability. Lex Luthor (Michael Rosenbaum) never entirely trusted Tess and planted a bug in her optic nerve to keep an eye on her. By the time Lex went missing, however, Tess was in charge of LutherCorp.
Her primary goal when she arrived in Smallville was to head the search for Lex. Tess suspected that Lex's old frined Clark Kent (Tom Welling), knew more about Lex's disapearance than he would admit. While investigating Clark, she simultaneously worked on another project: collecting "metahumans" who possessed special powers after being exposed to kryptonite.
When Lex's ex-wife, Lana Lang (Kristin Kreuk) arrives, she informs Tess that she knows where Lex is, and also tells her about the bug in her optic nerve. Tess angrily threatens Lex, jams his transmissions, and sets out to destroy him by selling out his company to his rival Oliver Queen (Justin Hartley).
Tess discovers Clark's true identity while reading Lionel Luthor's (John Glover) journals. She tries to persuade Clark to share his secret with her but he refuses. She becomes involved with the Kandorians, clones of former residents of the planet Krypton as she believes they will help save Earth from the destructive forces wielded by humans.
Tess passionately follows her beliefs and fights anything she sees as a threat to Earth, including humankind. She displays violent tendencies, such as poisoning and shooting people, among other crimes. Lois Lane calls her the "ultimate eco-terrorist" although at times she does show a vulnerable side to her personality.
"The mark of a true hero is somebody who's willing to sacrifice his own personal morality to help keep the world safe."
"The word 'fate' is used by people who've lost sight of who they are."
"One of the most important things that Lex taught me is you can't afford to live in the past."