Muscle Man

 Muscle Man is one of the groundskeepers at the park.

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 main characters, 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
Out of the two main characters, Rigby seems to like Muscle Man. Rigby 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.

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 Employees
There is minimal to no interaction between Muscle Man, Skips, and Pops. They had to take care of Rigby in "Death Punchies."

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 helped 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 catch phrase for getting hurt is "Oh no bro."Seen in Death Punchies and Rage Against the TV.