- "You want to know the truly perverse part? I'm upset that he threw me away... As grotesque and sordid a family tree as I've just inherited, I... Wasn't good enough." - Tess Mercer to Clark Kent, Luthor
Tess awakened in Cadmus Labs, mysteriously returned to life, with her face completely healed. She met "Alexander" (a conscious pre-pubescent clone of Lex), who told her how she survived. Tess discovered a multitude of clones engineered from Lex's DNA, and accidentally released an older psychotic clone, who incapacitated and imprisoned her in the facility, knowing that Clark would soon come to rescue her. When Clark eventually appeared, Tess relayed the clone's message to meet at Riley Field and later, when she returned to the Luthor Mansion, she took "Alexander" to live with her.
With seemingly minimal difficulty due to her wealth and with Checkmate dismantled by Zod's attack, Tess was able to return to normal life of being the CEO of LuthorCorp and manager from the Daily Planet.
Continuing in her effort to gain Oliver and Clark's trust, Tess revealed that she helped Chloe to reset the systems of Watchtower at Chloe's request and also provided Clark with information regarding the assassin Deadshot, in order to help him to protect Cat Grant.
At the same time, worrying about Alexander, Tess hired a therapist to diagnose the child's mental state and was warned about his possible mental instability due to his high intelligence. After rejecting the treatment for Alexander that the therapist had suggested, Tess decided to keep the child with the hope of saving him and started to show some affection toward him and develops a mother-son relationship with him.
Tess was doing her best to insure that Alexander Luthor didn't evolve into the evil Lex that everyone knew, however it had seemed that it would eventually be inevitable. After Alexander reminisced about the memories that Lex had with Clark, Tess tried to convince him that Lex was evil, but he refused to believe her and reminded Tess about all the darkness in her heart, forcing Tess to vent her anger by hitting him. She regretted what she had done and begged him to let her help him but Alexander refused Tess’s help again and reclaimed the name of his "birthright": Lex Luthor. This made her realize that Alexander was beyond saving and decided to lock him in the mansion, waiting for his apparent death.
With her knowledge of computer science and Veritas secrets, Tess became a keen information breaker and chronicler in information that concerned The Darkness that was written for the Traveler to defeat.
Tess began to devote herself to her new role at the Watchtower and made some modifications to make it a more pleasant place, to the dismay of Oliver, who wanted to keep Chloe's memory in it. After she helped the team to discover the plan of Rick Flag and the Suicide Squad, Oliver finally decided to start trusting Tess and welcomed her to his family.
After Tess started to have nightmares about being a child and a music box that mysteriously appeared in the mansion, she told Clark and they tracked it down to an orphanage run by Granny Goodness. After Tess saw a photo of herself as one of the orphanage's prized alumni, Tess discovered the truth about her past and Granny revealed that she was the one who took her from the hospital when she was declared dead because she was her favorite girl and will need her in the war that was coming. Tess and Clark made their way out from the orphanage and later Tess went through her file from the orphanage and was shocked to discover that her real name was Lutessa Lena Luthor, the daughter of Lionel Luthor and Pamela Jenkins.
She worked along with Clark, Lois and AC to discover the truth about the Vigilante Registration Act and rescued Oliver after he was enlisted and later learned about the darkness that was coming to earth.
Finally, Tess began to take her position as legitimate heir of Lionel and was given to her a Kryptonian device that belonged to her father. When Clark discovered that Tess had been helping Alexander and began to question Tess's loyalty, she tried to explain everything to him, including that she was a Luthor, but failed because Clark disappeared in front of her eyes. Tess was ambushed by an alternative version of Clark from a parallel earth, but after the real Clark reappeared, sending back the evil Clark to his world, Tess finally revealed that she was Lionel's daughter and Clark supported her by saying that she should be thankful for not having been raised by him.
After hearing about Clark and Lois's engagement, Tess held a party for them at Watchtower. Later, Tess was escorted away by a government agent for questioning about the vigilantes. Tess was interrogated by Lieutenant Trotter about her relationship with Oliver and his team, and when she wouldn't give up any information, she and Emil Hamilton were taken to a warehouse to continue being interrogated but were saved by Clark.
When Tess found that her long lost father had returned to reclaim LuthorCorp from her and Oliver, Tess immediately recognized that it was the Lionel from the alternate universe. After Lionel tried to reach out to her, seducing her with the idea of being the family she never had, she learned that he wanted to find Alexander and informed Clark about Lionel’s arrival. After arguing with Clark about what to do with the Luthors men, the two were faced off by Alexander and Clark was able to convinced him that Tess really loved him. Tess resumed her relationship with the boy to help him and knew that the only way to save him was by killing him. Tearfully, she tried to inject Alexander with a dose of cyanide, as a mercy killing but the needle was bent when touched his skin and Tess was shocked to learn that Alexander was invulnerable.
Under the influences of a magical champagne, Tess got drunk, with the rest of the gang, and lost all inhibitions that she possessed. She mounted a show with Dr. Emil at the karaoke of the Ace of Clubs and also begun to forge an emotional bond with him, especially since the two had sex while drunk.
Finally, Tess told Clark that Alexander was renamed Conner Kent after it was discovered that he also possesses Clark's DNA and powers and as a way to protect Alexander from their father, she showed Lionel the “remains” of Alexander, but was unsuccessful in her deception. Trying to put an end to Lionel’s controlling influence, Tess managed to discover that Lionel overlooked her adoption paper at the orphanage and was able to prove that Lionel was not who he claimed to be. After had Lionel liquor traced with nanoscopic tracking devices, Tess confronted Lionel, regained control of LuthorCorp, and had him escorted from the building.
While trying to sell the remains of the Luthor Mansion, Tess was ambushed by Clark Luthor who returned to kill Lionel and wanted her help to locate him. Tess was placed on a pedestal of having to choose between giving in to her instincts and desires or do the right thing, revealing that she had feelings for Clark and wanted revenge on Lionel for abandoning her. After hesitating for several moments, Tess finally chose to protect Lionel because he was her father and when she was about to be murdered, was rescued by the real Clark. All of these events led Tess to put into question again her side on the battlefield.
After Tess learns that General Slade was found unconscious on a street corner, she informs the league and send Clark and Oliver to the Phantom Zone to investigate of what is going on there. Tess was approached by Lois to get an update on what was going on and Tess began to be reluctant about sharing information. After Lois discovered that Tess had planned to destroy the gateway of the Zone from the inside, she threatened Tess with a gun to stop her plan as Tess did not hesitate into let her friends trapped under the guise of the greater good. Tess then revealed that she was only following Clark´s orders and once the two had returned, Tess helped Oliver on his quest for finding the Bow of Orion and provided him with information.
When Stargirl was controlled by a mysterious device, Tess analyzed the mind control diode at the Watchtower and discovered that it was created by none other than the Toyman, Winslow Schott, for the villains of Marionette Ventures. Later, Tess and Clark received news from Courtney that their transportation to Stryker's Island was ready, just as a super-powered, mind controlled Lois arrived at Watchtower and attacked Clark. Tess, in a panic, counted down the seconds to sundown when Clark would get his powers back, hoping that he could stall Lois until then.
One morning, Tess was at the remains of the Luthor Mansion seeing the plans for the new project she would build on the site to help ease the Luthors history in Smallville when she was approached by Granny Goodness in a last attempt to recruit and "save" her. After refusing again to serve the dark, saying that she had decided to stay in the light, Granny reveals that Apokolips was approaching to earth and sadly gave her blessing to Lutessa.
Tess went to the Watchtower to see if what Granny Goodness had told her was true, but was surprised to discover that she could not access the satellites of the team. After investigating what was happening, Tess found that Oliver had disconnected the satellites to prevent them from noticing the arrival of Apokolips. Using the cameras of the space station that John Jones has just donated to the team, Tess discovered that Apokolips was above them and immediately went to tell Clark.
On her way to Smallville and because the communication signals were interrupted by Apokolips descent, Tess could not reach Clark and then she began recording a video to him, but in the process, she was hit by several armed men who kidnapped her.
Tess wakes up very confused in a laboratory and discovered that was Lionel who kidnapped her. He informed her that he had found a perfected clone of Lex, who was compromised with the parts of the flawed clones and revealed that he planned to use her heart as the last piece to resurrect Lex. Tess was quite scared when Lionel told this and try to escape but was stopped by Lionel's thugs.
Before beginning the surgery, Tess begged for her life again but without success and before breaking free of her restraints, shooting her "father" in the chest before escaping.
After investigating the laboratory where she had been captive and finding no traces of Lex, Tess returned to LuthorCorp only to meet him in her office.
Lex reveals to her that he knew they were siblings from the beginning and that's why he was so confident in her. Tess revealed that nothing of that mattered now because she had not yet found her redemption. Lex stabs her in the abdomen after telling Tess he loved her. As she dies in her brother's arms, Tess touches Lex's face, affectionately at first, then wipes a neurotoxin on his cheek, a biochemical that removes all of Lex's memories up until that point. Lex told Tess that he was saving her from becoming like him, though Tess proclaims that Clark already has. Tess then died, finally achieving the redemption she'd always wanted.