Flex is a one-time villain from Teen Titans Go!, only appearing in the episode "Birds". He and Muscle are a pair of mutated mockingbirds, who often antagonize the Teen Titans.
He was voiced by Tom Kenny.
The chimney to the Titans Tower was clogged and Robin had to clean it out. He spotter a pair of mocking birds, Flex and Muscle who had built a nest in there. As one would expect the two mockingbirds started mocking him. Robin threw an exploding can of birdy-cide poison in there, forcing them to evacuate. The birdy-cide did more than just scare them off. The chemicals mutated the birds and turned them into two giant mutated mockingbirds.
Flex became blue and started acting like a jock, much like his orange friend, Muscle. He also gained the ability to speak English and live among humans. The Teen Titans met these birds and became friends with them, admiring their strength and gun shows. When Robin showed up, the two instantly recognized him and started hating him, calling him out for throwing them out of their house, earlier.
Flex and Muscle partied with the Teen Titans in the Titans Tower and Flex hit on Starfire multiple times. Starfire actually kind of liked Flex back as she giggled lovingly at all of his passes at her. Robin kept on invading the party to try and kick the birds out, but eventually, the birds and the other Titans had enough and they kicked Robin out.
It did not take long before Flex and Muscle started to annoy the other Titans, as they started throwing loud and rousing bird parties in the house, which they did not clean up after. Flex started hitting on other female birds, much to the jealousy of Starfire and he even started molting his feathers off all over the couch, annoying Starfire even more. He and Muscle also threatened to beat up Cyborg for eating an egg sandwich.
The Titans received another crime alert, saying that the chimney was clogged once again and the mockingbirds made the Titans go up there and do the sweeping. The Titans found Robin up there, living in a nest of his own. Robin explained that this is where he began living now that he'd been kicked out and the Titans agreed it was time to let him back under their wings and break the wings of those mockingbirds.
The Teen Titans broke back into the Titans Tower and fought with the mockingbirds. Unfortunately, the mockingbirds were too powerful for them to defeat. Robin had one last trick up his sleeve and that was to throw another exploding can of birdy-cide at them, hoping that it would kill them. However, all this did was mutate them even more, to the point where they became extremely intelligent, future birds, who had evolved millenniums past their former selves. Flex and Muscle thanked the Teen Titans for turning them into such ingenious creatures, even if it came at the expense of them losing their muscle mass as their gain in brain mass was even better. The two surrendered, saying they have given up evil and would no longer fight the Titans. Robin saw this as a chance to beat them up and win. Robin attacked both of them, brutally and the birds were powerless to stop them as they lost all of their physical strength in the next mutation. The two mockingbirds flew away in cowardness, as Robin cursed them and claimed victory.