Number Six (voiced by Luke Arnold) – Vice Commander of the Tech Men.It is revealed that he is the leader of the Tech Men who was aware of Rook's double life. Number One / Malcolm Kane (voiced by Keith David) – Malcolm Kane is the head of security for Rook Unlimited and the former mentor to the Flex Fighters.After the Epsilon Society was disbanded, some of its members came back together as the Tech Men where their cult-like aura had them believing that they are the true handlers of technology. At an earlier life, Jonathan Rook wanted to join the Epsilon Society only to change his mind when he didn't want to sacrifice his family and his identity. Tech Men - Formerly known as the Epsilon Society, they are an evil organization.Near the end of season 2, Stretch Monster had to work with the Flex Fighters to fight a common foe in the Tech Men. Unbeknownst to them until the season 1 finale, Rook is secretly Stretch Monster, an elastic creature who is also a benefactor to the criminal kind. Stretch Monster / Jonathan Rook (voiced by Wil Wheaton in human form, Miguel Ferrer in monster form in 2017, David Kaye in monster form in 2018) – Jonathan Rook is the CEO of Rook Unlimited and the benefactor behind the Flex Fighters.Dreamscape' and she follows them in 'Riya's Revenge.' She later gets energy powers and a purple energy form capable of flight in the season 2 finale. She becomes the team's ally after Nathan tells her the truth in 'Dr. Erika Violette (voiced by Felicia Day) – A classmate of the Flex Fighters and Nathan's love interest.She later reveals her parents worked on the Flexarium formulas before Stretch Monster killed them by sabotaging their plane to crash on the Pearl Islands. Blindstrike / Riya Dashti (voiced by Nazneen Contractor as a teenager, Kathreen Khavari as a child) – Riya Dashti is an Iranian-American student and classmate of the Flex Fighters who is secretly a Ninja warrior working with Dr.She faked her disappearance in hopes to stop her former protegé from taking over the world. Racine Cleo is a scientist who mentored Jonathan Rook. C / Racine Cleo (voiced by Kate Mulgrew) – Dr. Like Jake and Nathan, Ricardo is also exposed to the same experimental chemicals, and gains the ability to change his size and mass. Omni-Mass / Ricardo Perez (voiced by Ogie Banks) – Ricardo Perez is the new, hotshot kid at Jake and Nathan's high school whom they befriend.Wingspan / Nathan Park (voiced by Steven Yeun) – Nathan Park is a high school student with a large family and is Jake's best friend who is also exposed to the same experimental chemicals, and gains the ability to flatten his body from the neck down to gain gliding suit-like wings.

Stretch / Jake Armstrong (voiced by Scott Menville) – Jake Armstrong is an over-scheduled high school student who is exposed to experimental chemicals that gives him super-stretching abilities.They must also deal with Malcolm Kane when the Tech Men attack. C and Blindstrike to fight Rook's mercenaries while working to clear their name. In season two, now that Rook has fooled all of Charter City into thinking that the Flex Fighters are now bad guys, the Flex Fighters work with Dr. C and Blindstrike in order to expose Rook's crimes and clear their names. With the city under Rook's control and its citizens being turned against them, the Flex Fighters decided to join forces with Dr. C and Blindstrike, the Flex Fighters escaped from Rook, but he then frames them in public for the rampage he caused over the city. C's 'HyperFlexarium' formula to create an army of monsters and conquer the world. In their last mission, the Flex Fighters discover in horror that Jonathan Rook and Stretch Monster are the same person and therefore, he has been their enemy the whole time. C (Rook's former mentor) and her ninja companion Riya Dashti / Blindstrike (Stretch's classmate and love interest). In their first days as superheroes, the Flex Fighters receive hard training from Malcolm Kane (Rook's second-in-command) while fighting numerous villains including Stretch Monster, who shared a past with Dr. Agreeing on the terms that no one will know their real identities, Jake becomes Stretch, Nathan becomes Wingspan, and Ricardo becomes Omni-Mass. Rook offers them their freedom from the authorities over the accident in exchange for enlisting the trio as Charter City's superhero team known as the Flex Fighters. Their newfound superpowers garnered the attention of Jonathan Rook, the CEO of Rook Unlimited and benefactor to Charter City himself. Jake Armstrong, Nathan Park, and Ricardo Perez were three ordinary teenagers living in Charter City until they were doused in a substance known as 'Flexarium' during an accident.

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