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.  


Season 1

In Let's Call It Mech-X4!, Seth becomes intrigued in Mech-X4 and wants to know more information about it.

In Let's Get Some Air!, Seth (as the Mastermind) discovers there are humans on the robot and questions Leo (who's held him as captive) if he knows these humans.

In Let's Open The Monster Heart!, Seth catches Ryan, Harris and Spyder using his laser. He makes them leaves and he questions them why they were at Harper Futuristics. They had to leave due to the alarm ringing. When they got out, Seth asks Grace on a date.

In Let's Be Idiots!, Seth and Grace went out for sushi. When they return home, Mark told Seth about going to Rat$z, but couldn't because he didn't have a ticket. Seth said that he could get him to the concert. Seth (as The Mastermind) was getting tired of waiting and wanted results. Seth tried many different ways of trying to get into the concert however they didn't work. Seth and Mark decided to make their own Rat$z concert and Grace comes in to join. Seth (as the Mastermind) is informed by Principal Grey that she knows where Mech-X4.

In Let's Survive in the Woods!, Seth (as the Mastermind) got told by Principal Grey that Principal Grey knows the pilots and that if she has the robot.

In Let's Get Our Robot Back!, Seth (as the Mastermind) came in a car to see Mech-X4, but the robot shot missiles everywhere causing Seth to retreat. Since that happened, Seth planned on eliminating Grey.

In Let's Deal with Our Stuff!, Seth wanted Cassie to announce his scholarship, Harper Grant for Young Futurists. Cassie agreed and interviewed everyone, including Mark and Harris about it. Seth visited Ryan and Mark to check on them. He played video games with Ryan. Seth talked about with Ryan that Ryan and Mark's relationship isn't good. Seth and Grace and at the game and Seth implied that he made Ryan cheer for Mark.

In Let's Get Some Answers!, Seth (as the Mastermind) ordered his guards to give Leo lunch. Seth tried to get information on who the pilots were through Leo. Leo got re-captured and Seth told Leo that he's going to be trapped in that room until Leo tells him who the pilots.

In Let's Get Leo!, Seth (as the Mastermind) reminded Leo that nobody will find him so he should just tell him who the pilots were. Seth went to Harris' dorm and heard Harris and Augie talking about that Harris was doing something important and offered to turn the electricity back on, but Harris declined the offer. Seth made the other children at Harper Grant jump their ideas free. Harris faked hurting his knee and allowed him to go to the nurse. Seth (as the Mastermind) got told by his guards that Ryan was at his base.

In Let's Dig Deep!, Seth was glad that Ryan and Mark came to Harper Futuristic. Ryan tricked Seth and made Seth wear the device. Whilst Ryan was in Seth's brain, Seth saw what was happening and could control what was happening in his mind. He fought Ryan and was about to kill him, but Ryan took the remote and escaped. Seth woke up and called Morris to tell him that he knew who the pilots are and where they are going.

In Let's Destroy Some Ooze!, Seth gave Morris the Chameleon DNA to create a monster. Seth fought Mech-X4 using the monster he gave to Morris to plant. Seth had a Plan B and made it look like the monster was injured. Seth controlled the invisible monster to move which made everybody think that Mech-X4 shot missiles at Bay City High. Seth was with Grace and was planning to take the ooze after the military destroyed Mech-X4.

In Let's End This! Part One, Seth revealed to Grace that he was behind the monsters. Grace tried to escape, but Morris captured her. Seth built a missile to destroy Mech-X4 for the military. Seth found out that the military was about to use his missile to destroy Mech-X4. Seth controlled another monster inside to fight Mech-X4 himself.

In Let's End This! Part Two, Seth fought Mech-X4 whilst in the monster. Seth left and waited for Grace to join him. Ryan, Harris, Mark, Spyder and Leo made a plan to stop Seth and the monster. Seth managed to get free and went to Harper Futuristics to ask Morris if the "monster heart" is ready to launch. Grace got the trigger and she was able to fight the monster inside her. She dropped the trigger and Seth got it back. He activated the trigger and he got back into the monster so Ryan won't stop the Primorphous Core. Ryan threw the missile back at Seth, but he used his escape pod to escape from the ooze.

Season 2

In Versus The Deep, Seth was held prisoner by Kim Grey. Kim told Seth that she didn't get the Primorphous Core, but got a component she needed. Seth was the one who gave Ryan the visions. He was next going to do another one in person instead.

In Versus The Outbreak, 2 weeks earlier, it was confirmed that Grey found Seth and used him to make a similar gel that Grey fell into to give Traeger the ability to turn into a monster.

In Versus Harper's Ghost, Seth was making more ooze and was planning a monster by Grey's orders. Seth tried to send Ryan a vision so that his team could rescue him from Grey. Ryan, Harris, Mark and Spyder realized he was trying to say neurosplicer. Ryan, Mark ,and Spyder rescued him and the monster called Arachno-Rhino (named by Seth) fought Mech-X4. Ryan brought Seth so he could erase his mind using the neurosplicer. Seth escaped when Ryan was going to send him to jail.

In Versus The Tech Army, Seth built a robot's arm and controlled it with his fingers and it worked.

In Versus The Wolves at the Door, Seth was in Jaguasaur. He came to help Mark, Harris, Spyder, and Veracity get to his Arctic Base. Mark decided that they should let Seth help find Ryan. Seth, Harris, Mark went to find Ryan in the upper section of the robot. Seth, Harris, and Mark split up so they could find Ryan. Seth turned the internal sensors back online. Seth found Ryan and told Harris, Mark, Spyder, and Veracity. Seth was trying to get Ryan under control but he saw Seth as Traeger. Seth showed Ryan the robot he was making and he was planning on using those parts to build an upgraded Mech-X4.

In Versus The Thirty, Seth was helping Harris, Mark, Spyder and Veracity re-attach Mech-X4. Seth left to go to the lab. Seth met Ryan. Ryan reminded Seth that he wasn't an official member of the team. Also, Ryan reminded him that they were still enemies after Traeger was gone. Seth carried on repairing Mech-X4 after his talk with

In Versus Miami, Seth and Leo were trying to fix the X-Weapon but they were arguing. Mark and Veracity tried to make them stop, however, it didn't work. Seth and Leo stopped arguing when Ryan stopped them. Leo electrocuted Seth. Seth woke up and found himself captive by Leo, similar how Harper held Leo captive. Harper escaped and wasn't planning to tell the team about him making monsters with him. After that, Harper and Leo put the power back to normal.



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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