Muscle (real name unknown) is a professional supervillain henchman. He is hired by Hexagon to help rekidnap Headrush. There is a rumor that he fought Mimeo when both of them were young and new. He fights Imp and Superhawk when the rescue Headrush the second time. He recommends Imp for a job with another employer, but Imp turns him down, as she is already busy with rescuing Mischief.
Muscle is a Shifter who could add mass to his body, increasing his height, musculature, and density a great deal. Once he reached seven feet...and he was so bulging with muscles that a world champion bodybuilder would be puny in comparison, he stopped growing, though I knew from personal experience that he could grow another foot if he wanted. When Muscle was fully pumped up, he was probably about the equivalent of an exemplar 4.