Flex Mentallo was created by the writer Grant Morrison. His first appearance was in Doom Patrol #35.
Flex Mentallo was originally a nobody. Sick of being the weakling, he sent in a form that would teach him the powers of muscle mystery. This did indeed grant him incredible muscle power, and he was granted superpowers through the sheer power of his muscles. For a time he was part of a group which included such esoteric members as the Atomic Pile, who was half-man, half-salad, and the Fact, who had little cards with facts on them.
Life went on until Flex discovered a conspiracy in the Pentagon. Afterwards he couldn't remember what it was, but he knew it had something to do with the five sides. He assumed that by turning the Pentagon into a circle he would solve the problem. But flex as he might, the Pentagon refused to change. He lost his powers, and was lost for a time, his memory disappearing from him.
Eventually he reappeared on Danny the Street. He slowly regained memories of what had happened, and recruited the Doom Patrol to help him stop the conspiracy. It was revealed that the Pentagon was home to the evil Telephone Avatar. The plan was to make the whole world like a giant machine, accomplishing nothing, but with each piece in its place. Creating ideas for the Pentagon was Wally Sage. He was a powerful psychic, and had created Flex Mentallo when he was a child, and had him star in his own series, 'My Greenest Adventure'. With his dying breath, Wally gave Flex back his powers. Knowing what he had to do, Flex Mentallo flexed with all his might. This time he succeeded, and the Pentagon became a circle. The Telephone Avatar was depowered, and Dorothy Spinner called upon the Candlemaker to destroy it. His task complete, Flex went his separate way.
Flex later returned to Doom Patrol, assisting them with various problems inside the world of 'Dannyland', a gender queer sentient realm.
The Patrol assists Flex with his own problems, namely Muscle Beach becoming run-down and over-run with bullies. Flex and the Patrol solve this situation. Rita Farr is an essential part of this plan, as she learns her own size-changing abilities allow her to flex in various helpful ways. The efforts of the team makethe Beach, once again, a fun place to hang out in and build your body.
Flex Mentallo mini series
In his own limited series, Flex Mentallo had to discover a conspiracy where his own fictional reality is colliding against the real world where he now resides, meanwhile Wally Sage, now depicted as a successful rock star akin to Kurt Cobain, remembers his past confusing about what is real and what is fiction in the middle of a suicidal crisis.
Flex starts searching for his old teammate, the Fact, with the helped police officer Lieutenant Harry. Flex discovered than the superheroes of another universe, The Legion of Legions (composed by several homages to Marvel and DC characters) had became fictional to survive the attack of an unstoppable force known as the Absolute and had seek refuge in the unconscious mind of Wally Sage, who was also the villain who wanted to show the world than Flex wasn't as virtuous as he claimed. Flex was able to resist the attack of Wally and helped him to overcame his cynism and bring the superheroes to real life.
The miniseries touch concepts as metafiction, fourth wall ideas and concept seen usually in Grant Morrison's books.
Flex appears on the DC streaming show, Doom Patrol. He is a classic superhero who has been imprisoned by the Bureau of Normalcy for some time. He is busted out by the Doom Patrol and teams up with them to find their leader, Niles Caulder.
Flex Mentallo has the powers of Muscle Mystery. By flexing his muscles he is able to do an astounding number of things, including summoning his 'Hero of the Beach' halo.