Have you seen that guy's hands? They flop around like a goldfish out of its bowl!
Spyder about Seth in Let's Dig Deep!

Seth Harper is a character in MECH-X4 who is the rich head of Harper Futuristics. In Let's Get Leo!, it's revealed he's The Mastermind, Principal Grey's boss and the one behind the monsters, serving as the main antagonist of Season 1. He returns to taunt and bedevil the team in Season 2. He is portrayed by Peter Benson.


Seth Harper is the billionaire CEO of Harper Futuristics, a company centered on the development on future tech, as well as genetic engineering. Back in the day, he and Leo Mendel were good friends. He was disgusted at the harm people have caused to the environment, that by eventually founding his company, he could change things. Secretly, he assumed the role of The Mastermind, with many of his subordinates in his secret operation not knowing his real identity. He was behind the genetic engineering of monsters from a unique substance known as Primorphous Gel. Upon learning Leo created MECH-X4, he saw his old friend as a threat to his agenda and captured him. As the Mastermind, he dispatched Principal Grey and other agents, such as Janitor Morris at Bay City High. Through his agents, they would unleash monsters to lay waste to humanity and make way for a new paradise on Earth.


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 as well as their selfishness in building weapons of war. From that he became bent on sending genetically engineered monsters to wipe out humanity, and only those he deems worthy will be spared.  


Seth started up as a college mate with Leo. Leo gave him an idea of changing the world which made Seth explore. When he explored, Seth found a cave of Primorphous Gel. Harper tried to track down Randall but he couldn't find him. Seth trapped Leo but Ryan got him out.

He and a few other people managed to make monsters so that they could ruin the world so that Seth could restart it because of pollution. Seth made an industry including Harper Futuristics that kept his private information about monsters.

In Let's End This! Part Two, Seth took Grace and he wanted to make her apart of his "new world". Ryan got her back and the team defeated the monster. He was capable to eject before the defeat and landed in the woods. Grey found Harper in the wood and enslaved him to make monsters and more Primorphous Gel. Ryan got visions about Harper in Versus Harper's Ghost and was going to erase his memory. Harper manipulated him and he was able to leave.

When Harris, Spyder, Mark and Veracity lost the robot and thought they lost Ryan, Harper helped them in Versus The Wolves at the Door. Ryan and Harper formed an alliance until Traeger is defeated. After the team supposed Traeger was destroyed, in Versus The X-Weapon, Seth left a note saying he left.



Grace Walker

Grace is Seth's romantic interest. They have gone on two dates so far and Seth has been seen at her house with her in Let's Be Idiots!. It is later revealed he was using her as leverage against MECH-X4, though she is oblivious to it. However, he wanted her to be a part of his new world, but he brainwashed her with monster DNA. They later broke it off when it was revealed he was the one behind the monsters.



Harris has been shown to be a fan of Seth since Let's Call It MECH-X4!, and Seth says he likes him in Let's Open The Monster Heart!, since was daring and initiative. However, once Harris finds out he's the one behind the monsters, his opinion of him has changed.

Mark Walker

At first, Mark doesn't like Seth and isn't impressed by his wealth, intelligence and money. But after Seth does his best to get Mark into a DJ Rats$z concert without bribing anyone, the two become friends. However, after finding out that Seth is in fact the one behind the monsters, Mark's opinion of him changes.

Ryan Walker

Ryan does not like Seth at all, after finding out he's the one unleashing the monsters and has been messing with his life since he was born, even going so far as to having Ryan's birth parents go into hiding. In Let's Destroy Some Ooze!, Ryan shows how much he hates Seth, saying he just wants to break into his penthouse through MECH-X4 and take him down.


Not much is known or mentioned about Seth's relationship with Spyder, but in Let's Open The Monster Heart! Seth says he does not like him and after the events of Let's Get Leo!, it's assumed Spyder does not like him either. It's later evident that out of all the kids, he hates Spyder the most. Seth's hatred of Spyder was used against him as a distraction.


Harper hates Traeger because he turned Kim Grey against him and kept him captive for weeks. He and the Mech-X team have formed an uneasy alliance to take the mysterious figure down. However, Traeger, himself, hints that the two may have a secret connection that Harper apparently has no awareness of, as he reveals to Grey that they are "brothers."


Season 1

Season 2


  • He invented a laser that can cut through anything.
  • He is a fan of MECH-X4 and wants to learn more about the robot.
  • Seth has a hard time not using his money to solve problems.
  • Seth's relationship with Mark is rocky.
  • Seth is a very complex and interesting villain. It's said in Let's Dig Deep! that he wants to destroy most of humanity in order to make the world a better place, due to the fact that people keep trashing and polluting it with toxic waste, garbage and etc. In fact, he's so disgusted by acts of selfishness, that he's willing to commit terrible acts in order to achieve this nobler goal for humanity.
  • According to Godfrey in Versus The New Evil, Seth "died" at 29.


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