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Many years ago, he was good friends with a fellow genius known as Leo Mendel. Back in the day, he was not happy that a CEO polluted the river, and when he came to speak at his college, he genetically modified the dean's ficus plant to attack the CEO. He swore one day, he would have the power to change things. That inspired him to become the founder of Harper Futuristics. Harper grew disgusted at the pollution humanity was bringing to the environment, and made it his goal to wipe out humanity and only those who are worthy would live in his new paradise. Using Primorphous Gel and the DNA of various animals, those would be used to create monsters to lay waste to humanity. Upon learning Leo created MECH-X4, Seth saw his old friend as a threat, and held him prisoner. To carry out his agenda, he assumed the codename of "The Leader", and many of his followers including Principal Grey never knew his true identity.
Harper later discovered a large reservoir of Primorphous Gel under Bay City High school and dispatched Principal Grey to watch over the gel so that he can later destroy the school and harvest the gel. In the meantime, Principal Grey would send various monsters to carry out their destruction, but things changed after Ryan Walker and his friends discovered Mech-X4, leading to his curiosity about the robot.
Since then, Grey has been trying to eliminate the robot, but after costing him a supply of Primorphous Gel from a volcano and being thwarted many times by the crew, Harper grew impatient and was about to eliminate her until she mentioned the robot had at least 3 human pilots. Harper was intrigued, and Grey agreed to bring him the robot. While Grey hired a hitman undercover as a park ranger to take care of Ryan, Spyder and Harris, she sent a flying monster to hijack MECH-X4, and agreed to meet Harper in a camp populated by his soldiers. Once the kids came to recover their robot, Harper was about to arrive, and saw Grey did not have the situation under control and left. From then on, he decided Grey needed to be eliminated. Meanwhile, Grey attempted to team up with the pilots of MECH-X4 to go after him. After Grey contacted him one more time to meet him after school. Seth sent his right hand man Morris to terminate her, and brought a squad of soldiers to finish the job. In doing so, Ryan used Morris's phone to track his location. MECH-X4 arrived at the warehouse with his presence confirmed, but it turned out to be a trap. One of his monsters came out and ambushed them, and Grey tricked the crew into helping her save face with the Leader. Once Grey told The Leader one of the pilots was a technopath, he ordered her to capture him and deliver him. Unbeknownst to him, the team saw the Principal's double cross coming, and fought Grey, causing her to fall into the Primorphous Gel left over from the defeated monster. The Leader got away afterwards.
With Principal Grey's failure, The Leader decided to take a more direct approach and continued torturing Leo to tell him the identity of the pilots. Later on, Ryan, Mark, Harris and Spyder discovered Leo's location was in a deep sea lair. Ryan later broke into the lair and disabled the cameras, in doing so, the Leader knew he was in a technopath's presence, and attempted to kill Ryan by flooding the lair. When Ryan was being contacted, he traced the source and discovered that the Leader was none other than Seth Harper, CEO of Harper Futuristics.
After knowing the Leader's true identity, the pilots of Mech-X4 were determined to know where the Primorphous Gel is, so Leo came up with an idea to build a neuro splicer, a device that could connect Ryan to Seth's brain. Since Seth still thinks that they're his friends, the kids planned to use the device on Harper during his product launch.
At Harper Futuristics, as Seth was getting rid to deliver his speech, he welcomed Ryan and Mark as they came in
Seth is a very intelligent and curious guy with a deep passion for science and inventions. He is also shown to be great at inventing and puts others first when it comes to what's right, as shown when a fire (which later turned out to be fake) started inside Harper Futuristics. He is also caring and is willing to put effort into things, as shown when he wanted to improve his relationship with Mark, since he's dating his mom.
His largest conviction is his strong passion for the environment, but because of this, he became disgusted at humanity for polluting the environment. From that he became bent on sending genetically engineered monsters to wipe out humanity, and only those he deems worthy will be spared.