Muscle Man

 Mitch "Muscle Man" Sorenstein is one of the groundskeepers at the park.

Personality and Traits
Although thought of by Benson as 'hard working', Muscle Man (along with High Five Ghost) are slackers, equally or worse so than Mordecai and Rigby. He (along with High Five Ghost) have been revealed to do more work than Mordecai and Rigby in "Muscle Woman", though they tend to slack off more than Mordecai and Rigby. In "My Mom", it's shown that he has a lot of friends, most in which love his constant "My Mom!" jokes, to which both Mordecai and everyone else at the park get annoyed with. He's best friends with High Five Ghost, who loves his jokes. Ignorant and very good when it comes to pranks, he's 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 and brought his brother, John, to gave Mordecai and Rigby a punishment for ridiculing their mother.

High-Five Ghost
High-Five Ghost is to Muscle Man as Rigby is to Mordecai. They are usually seen together in the episodes. He usually gives a high five right after Muscle Man says a "My Mom" joke. In "My Mom!," the two seem to slack-off as much as Mordecai and Rigby.

Mordecai
Out of the two protagonists, Mordecai seems to dislike Muscle Man more. He is annoyed by the constant "My Mom" jokes and even confronts Muscle Man about it in "My Mom!". Besides that episode, there is not much interaction. Muscle Man does throw his shirt at Mordecai in "Caffeinated Concert Tickets", but that may have been an accident rather than a direct confrontation. He may have been doing this just so that they would not have to do his work.

Rigby
Rigby seems to like Muscle Man slightly more than Mordecai. He likes to hang out with him in "My Mom!", and seems to get along with him very well, yelling "Ohhhhhh!" along with him at the taco truck. They do have some confrontations though. In "Meat Your Maker", he goes to say Muscle Man pushed him at a party (though Mordecai quickly says Muscle Man was not even there). Also by the end of "My Mom", Rigby goes on to say how he hates Muscle Man. Also in "Death Punchies", Muscle Man is forced to be look out, causing his eyes to burn. They also hang out in "The Night Owl".

Benson
Benson trusts Muscle Man far more than Rigby and Mordecai, as established in "Appreciation Day" and "My Mom". He does fire Muscle Man though in "Don", but this may be due to the stressful situation that eneryone was in. Benson also seems to understand Muscle Man and his antics, as demonstrated in "But I Have A Receipt" and "Free Cake", not yelling at Muscle Man when he put "My Mom" on the list. He does not confront Muscle Man when he unzips his pants in "Free Cake", saying, " It's not that kind of party, Muscle Man."

Other Characters
There is hardly minimal to no interaction between Muscle Man and Skips and Pops, and many other characters.

Episodes that Muscle Man doesn't appear in
The Power

Meat Your Maker (mentioned)

Grilled Cheese Deluxe

The Unicorns Have Got To Go

Prank Callers

Ello Gov'nor

High Score

Really Real Wrestling

A Bunch of Baby Ducks (mentioned)

Skunked

Trivia

 * In the episode Muscle Woman, his real name is revealed to be Mitch Sorenstein.
 * Muscle Man also appears to be drawn similarly to characters from "The Marvelous Misadventures of Flapjack", a Cartoon Network cartoon J.G. Quintel worked on before starting Regular Show.
 * More often than not, Muscle Man removes his shirt during the episode he appears in.
 * Muscle Man likes Fist Pump and drinks soda.
 * Muscle Man's voice tone turned lower after Caffeinated Concert Tickets.
 * Muscles Man's catchphrase for getting hurt is "Oh no, bro.", which is seen in Death Punchies and Rage Against the TV.
 * He was seen checking out Margaret as shown in "First Day", though they haven't interacted since.
 * In Over The Top Muscle Man lasted the longest in the arm wrestling match, which means he is very strong, and probably why he is called Muscle Man.
 * He thinks an internet sensation called Wedgie Ninja is funny.