He used to be assistant to Rex's true parents, Dr. Ancient and Dr. Cretacia, until he betrayed him by helping Dr. Z take control of the Time Ship, (Zeta Point). From then on, he worked with Dr. Z in repairing Zeta Point until it was fully functional. However, in the meantime, he was really planning to take over the island and force dinosaurs into an extreme evolution so they could withstand the meteor from the Cretaceous Period that had wiped out the dinosaurs, as he said in Metal Imbalance. He attacks the D-Team and the Alpha Gang with Black Tyrannosaurus, but the two groups work together to defeat it and he is sucked into the past.
Dr. Ancient and Dr. Cretacia meet up with him again, and this time, he is working with the Spectral Space Pirates. He is often seen beside Spectre or in the lab, but doesn't become important until Sheer tries to capture the Red Cosmos Stone. He then advises her to control Isisaurus, which she does. Later, he creates Gigas, Armatus, and Maximus for Gavro. Foolscap, and Sheer, respectively. However, he betrays the Space Pirates after capturing the Cosmos Stones in the D-Team's possession. He first defeats the dinosaurs of his own creation, then proceeds to fight Max with his Cryolophosaurus. After his attempt to get all the Cosmos Stones fails, he goes back to the D-Team's original time period, where he holds Reese, Dr. Owen, and Patrick, hostage. He is defeated one last time before finally siding with the D-Team, destroying the Black Pterosaur using the stones and with the help of the Pterosaur. However, he is left in a comatose situation after that, and is taken back to the future by the Alpha Gang. In the dub, when Max asks about Seth's situation, Rod tells him that although the healing is rather slow, Seth is recovering.
In the DS Game, he is the last Alpha Gang member to be fought before Dr. Z. He is the only one who uses only one dinosaur (Spinosaurus) to battle the player, instead of a full party of 3. But once he loses, he uses another Spinosaurus, which was altered, to never lose at Rock, Paper and Scissors, and beating the player unfairly leads to the awakening of the player's starter dinosaur (Awakened Carnotaurus for Rex, and Awakened Triceratops for Max). He uses Water Dinosaurs like Spinosaurus, Alpha Suchomimus, Baryonyx, and Alpha Irritator.
Seth appears in the manga. In the manga, he is given a larger backstory. In the manga, Seth is called Nopiece, close to Nopis, his Japanese name. He was a hero of the tablet, and in order to obtain it, he joined the Alpha Gang to use their resources. When Max came along with the tablet, Nopiece stole it from him, but Rex revealed another one he found once he defeated them.
When Max is trying to retrieve the tablet which Nopiece took from Rex, he instead gives one of the two to him and says he will battle him, and if he wins he can become the hero of the tablet again. Max surprisingly wins, but he doesn't have the time to claim the other tablet before Alpha Mountain starts to disintegrate.