|“||I am my brother's shadow.||”|
— Season 3 taglines regarding the CASTOR Clones
|Manner of Death:||Shot by Rudy|
|Affiliation:|| Project Castor|
|Family:|| Virginia Coady (adoptive mother)|
|Portrayed by:||Ari Millen|
|First appearance:||The Weight of This Combination|
|Latest appearance:||Transitory Sacrifices of Crisis|
Along with the other Castor clones, it is mentioned that Seth and his brothers were brought to Dr. Virginia Coady and the military, where they grew up and received training.
In the season 3 premiere, "The Weight of This Combination", Delphine shows Sarah video footage of Seth and Rudy abducting another Leda clone, Krystal Goderich. While Rudy is caught and imprisoned, Seth escapes.
In the episode, Seth attacks Mrs. S at her house in an attempt to obtain information on Professor Ethan Duncan and his original research. Mrs. S informs him that Duncan is now dead and his research is probably at Dyad. Seth also appears to be suffering from disorienting and possibly painful flashes. At the end of the episode, Seth liberates his clone brother Rudy from his basement cell.
After freeing his brother from his internment, Seth accompanies Rudy to a hotel where they sexually harass a young woman and take a sample of her hair. The next morning they are visited by Paul Dierden, their military superior. Paul administers a neurological test on both brothers while studying their responses. Seth does not do well on his test due in part to his neurological illness (a glitch in the Castor clone biology). Paul issues orders for both brothers to return to base immediately, which Seth loyally follows until Rudy disobeys and continues their mission to find the original Castor genome. Seth is physically affected by his illness, a fact that is noticed by his brother Rudy, whose motivations stem from finding a cure for his brother(s).
When Rudy confronts Sarah in Felix's loft, Seth stands guard downstairs before coming into contact with Cal. At first, Seth moves to attack but is intensely affected by painful flashes. He begins muttering nonsense about his past and repeating the questions to his failed neurological test. He collapses on the floor in pain. Rudy appears and after realizing that Seth is too far gone in his illness, mercifully shoots him twice, killing him.