He appeared frequently throughout Season Nine.
He was portrayed by Ryan McDonell.
Stuart Campbell began working for Tess Mercer as an assistant. At the Luthor Mansion, Stuart enters and shows Tess that he's pinpointed the locations of the missing Kryptonians. Later, Tess and Stuart investigated the room in which John Corben awoke in earlier, and saw that the people performing surgery in there were testing something and were possibly still monitoring the test subject to see if it would work. Tess concluded that Major Zod and his people were one step closer to getting their powers.
After Corben's presumed death, Tess examined the Kryptonite heart that served as John's life support, in awe of the power when Stuart reported pictures he has found of Kryptonian symbols that appeared around the world when Zod arrived. He explains that, while their definitive translations aren't known, the designs showed similarities to the Kryptonian words for 'blood', 'nobility' and 'family'.
He said hundreds of these symbols have shown up around the world, proving there were many more of Zod's people out there. Finally he showed her the House of El symbol burned into the ground, but with one difference: this Mark of El had a man lying naked in the middle.
At the Luthor Mansion, Tess talked to Stuart who informs her that there was a hacker trying to get through to their systems. Tess tells Stuart to fix the situation or she will have to have him terminated; Stuart looked concerned by this.
Tess remained inscrutable as he left to his task. Later Stuart was battling it out in cyber space, with Chloe Sullivan. Chloe was trying to access Tess' files, and Stuart tried to avoid "termination." Clark walked into the watchtower to check on Chloe's progress just as she finishes hacking the 100th firewall protecting Tess' servers.
Just when Chloe feels that she should be getting in, a video of Stuart begins to play, telling Chloe to just give up. He puts his hand in front of the screen. Chloe was only encouraged by this and uses the video to identify Stuart by capturing his fingerprints.
Chloe explains that Stuart has crazy computer skills and that she wants to use the information she can gather on him to recruit him away from Tess. Later Stuart walks down an alley and turns, noticing Chloe following him. Chloe tosses down an envelope, which Stuart opens; the envelope contains the information Chloe pulled up on Stuart. She reaches out to shake his hand, Stuart accepts.
Later Tess ordered Lois Lane to be taken from the hospital to a facility where she was hooked up to computer software by Stuart. Stuart used some machines to show images of what she was remembering on a screen. He asks if it is because she told her therapist that she was having flashes of the future and Tess looks surprised as she had never told him anything. He admits to sneaking a peek at what he was hacking and Tess tells him to establish boundaries.
Stuart was told to use Project Intercept for Tess to enter Lois' mind. After releasing Tess from Lois' memories of the future, Tess told Stuart to erase Lois' memories. Stuart is unsure, noting that if something went wrong, Lois could end up catatonic. Tess tells him not to argue and he reluctantly accepts her instructions, but then decides to pull Lois out, saying they were messing with her mind.
However, Stuart stood his ground and defied her order, for which he was punished with a bullet in his back. When Clark arrived, Tess lies to him, saying Stuart went rogue and that he kidnapped Lois, and she tried to stop him. But seeing Lois, Clark roughly pushes Tess aside, throwing her into a table and she falls unconscious. Emil and Chloe were able to get Stuart to the hospital, and treated.
Stuart spent five months in recovery after Tess had shot him. He was recruited by Checkmate and became their White Knight. He helped hack Watchtower in order to get more information that Checkmate needed on the Kandorians. He then pursued Tess who was on the run from Checkmate after betraying them.
After the Kandorians were found, Amanda ordered Stuart to abandon his search for Mercer and pursue the aliens. Stuart, wanting revenge on Tess for shooting him, disobeyed Waller to not search for Tess and did so anyway.