Mitchell "Mitch" Sorenstein, commonly known as Muscle Man, is a major anti-hero of the Cartoon Network TV series Regular Show, he's not really a villain in the series but he did do a bit of very bad things that made him look bad before mellowing out.
Muscle Man resembles a zombie with brown hair. However, he is stated to be a human, at least according to Mordecai in same episode which about Starla with same "skin condition" as Muscle Man, and it is due to his green skin condition, along with his girlfriend (later fiance) and previously his father (who is now deceased), that still make him more of a human-like, green-skinned humanoid. Because of his skin condition, and rarely unexplained power, he and his girlfriend turn into flying color fireballs in "The Longest Weekend" then they are "Humans". And also like her he have superpower to to carried heavier objects even they are "humans" then their other real humans aren't (Presumably) having that superpower.
He enjoys bragging, pulling pranks, and annoying Mordecai and Rigby. He is overall, by these actions, viewed as a bully. However, he is a total idiot since he willingly got fired by Benson, but this contradicts the later events that Benson actually lets Muscle Man have lunch during his work shift and gives him an Appreciation Day certificate, rather than Mordecai and Rigby, for "working hard", though it is possible that Benson gave Muscleman and Hi Five Ghost the certificates because he just didn't wanna give them to Mordecai and Rigby (instead giving the credit to MM & HFG).
Despite his annoying habit of pranking, the episode "Prankless" revealed that his pranks are actually important to the park. Benson tells about how one of Muscle Man's pranks saved the park from a war with a rival park. In the episode, Muscle Man quits after accidentally injuring Pops with his pranks, bringing the park in peril. Pops later persuades him to pranking again by injuring himself with a boxing glove prank (saying it was from the rival park). This episode reveals that Muscle Man is not such as bad person, as he enjoys pranks so long as they don't harm others. He also becomes far nicer as the series goes on, even risking his life to save the park many times.
Muscle Man is a human who works in the park along with Mordecai and Rigby. He is also quite unhygienic and scatalogical, reeking of body odor as evidenced in "The Night Owl". As of the episode "Gut Model", he has worked for the park for five years. Benson considers him to be hardworking, and indeed, although Muscle Man (along with Hi Five Ghost) has been revealed to do more work than Mordecai and Rigby, the fact is all four of them are slackers. In fact, in "Muscle Woman", they tend to slack off more than Mordecai and Rigby. In "My Mom", it has shown that he has a lot of friends, most in which love his constant "My Mom!" jokes, which everyone else at the park gets annoyed with. He is best friends with Hi Five Ghost, who loves his jokes. Ignorant yet clever when it comes to pranks, he is always either insulting Mordecai or Rigby, or trying to prank them (ex. "My Mom", "See You There"). But he does care about his mom as shown in "My Mom" when he got very angry when Mordecai and Rigby made fun of his mom. In response, he brought his brother, John, to give Mordecai and Rigby a punishment for ridiculing their mother. Their punishment was them to say sorry to their mom, (which was muscle man's butt squished together to look like a woman's face) kiss the photo and eat it.
Despite being obese and lazy, there is a story behind Muscle Man's nickname (as revealed in "Peeps"). In the episode, he said that he is losing muscle mass from the stress Peeps put on him, so he may have muscle mass. He also loves to mess around with Mordecai and Rigby as he considers them good friends despite their constant fighting.
Muscle Man has developed a massive ego that is seen throughout the series especially in "Exit 9B", where he purposely has the biggest signature. He also shows this whenever he takes pride in his pranks as he hates when people outsmart him. That said, while he takes pride in himself, said ego, along with his other negative traits, aren't as evident once he mellowed out.
In "Fortune Cookie", he switched Rigby's fortune, causing Rigby to switch Benson's fortune, nearly destroying the park.
In "Big Winner", he trashed the park and Mordecai and Rigby's house, thinking he will be able to pay for the damages with his fake lottery ticket money. He also damaged a lottery machine and attacked some police officers. Also, is revealed that he attacked a man named Biggs only because Biggs pulled a prank on him, and Muscle Man didn't find the prank as funny as Biggs did. The episode also ended with Muscle Man telling Mordecai and Rigby they will need health insurance when he pranks them back, implying he was planning to murder them.
In "Muscle Mentor", he tried to hurt Rigby by smashing him into things. He also mocked him, and put them both in danger. Therefore, while he redeems himself by expressing his pride towards Rigby for completing the program, he took things too far regardless of whether he was ordered to do this by Benson/
In "Starter Pack", he pranks Thomas and attempted to attack (possibly even murder) him after "he" (Mordecai and Rigby set up the Trash Bag Prank) pranked him back. It turned out this he and Thomas worked together to pull a prank on them. Even so, this prank destroyed a great deal of the park.
In "Firework Run," he got Hi-Five Ghost, Mordecai, and himself in trouble with Benson after wasting all the fireworks. They all would have gotten fired if that one firework did not top last year's show. Furthermore, his actions also put them and himself in danger.