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.
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 mentallo mini series
In his own limited series, Flex Mentallo had to discover a conspiracy were his own fictional reality is colliding against the real world were he now resides, meanwhile Wally Sage, now depicted as a sucessful 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 inconcious mind of Wally Sage, who was also the villian 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 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.