Mordecai's romantic life is the subject of several episodes, particularly his crush on Margaret Smith, a red-breasted robin who worked as a waitress at a local coffee shop.
In the epilogue to "A Regular Epic Final Battle", Mordecai quits The Park, parts ways with Rigby, moves to New York City, and returns to making art.
Eileen Roberts, a female mole who works at a local coffee shop, previously alongside Margaret Smith, has strong feelings for Rigby and makes this very obvious.
They eventually get married and start a family, and it is likely that Rigby is a househusband, because he is seen teaching his two daughters how to play video games, and he does not have enough skills to be employable for a decent-paying job, so Eileen would earn more money than him as an educated college graduate.
Pops Maellard (later revealed to be Mega Kranus; voiced by Sam Marin) is a cheerful and somewhat naive 6'2" lollipop man who resides at the house in The Park and always talks in a whiny-like tone.
He sometimes serves as a peacemaker during times of conflict, using his imposing stature and demeanor as a way of discouraging others from getting out of control in heated situations (though he is also prone to outbursts of his own on rare occasions).
In "Power Tower", it is revealed that he gained the nickname "Muscle Man" from his early years of bodybuilding and actually being in excellent physical condition, but he found that life very boring and gave it up.
Eileen Roberts (voiced by Minty Lewis, a writer/storyboard artist for the series) is a 3'8" anthropomorphic mole who works as a waitress at a local coffee shop, previously alongside Margaret.
In the episode "One Pull Up", Eileen helps Rigby train for the Park employees' physical test and he shows his gratitude by hugging her (and crushing her due to his enlarged physique).
Nikolai (seasons 4–7) (voiced by Roger Craig Smith) is a young anthropomorphic goat who is of Ukrainian origin and was trained by the KGB as a ruthless spy, and who later sent him to work for the Russian organization M.O.M.M.
In “Rigby's Graduation Special”, he makes a brief cameo by proposing a toast to the park dome taking off into space while in his submarine, hinting that he is aware of their mission and Pops's destiny.
After learning that Margaret was not getting married, he almost immediately forgets about CJ, leading to an ugly break up at the coffee shop in which she revealed that she can transform into a deadly storm when really angry and assumed this form in three episodes.
In the series finale, when the park gang returns from space after a three-year absence, Mordecai notices Mr. Maellard staring up at the sky in grief, shedding a tear for his late son.
Death (voiced by Julian Holloway in a cockney accent) is the grim reaper who, in several of his appearances, makes bets with park workers and declares that he would take their souls if they lose.
In the episode "Rigby's Graduation Day Special," he sent a fax to the gang to push the button to launch them into space for their mission: protect and train Pops against his long-lost evil brother, Anti-Pops.
Garrett Bobby Ferguson Sr. (GBF Sr.) (voiced by Sam Marin) is a giant floating head from outer space who is based on the video gamer Billy Mitchell.
The Wrassle Frassle Wrestlers (voiced by Lee Reherman and William Salyers) were the villains and minor characters who first appeared in "Really Real Wrestling", consisting of Forearmageddon, Hissy Fit and The Fire Marshall.
He is the son of video gamer Garrett Bobby Ferguson, posing as a Contractor to turn the Park into a highway for the purpose of invoking a summon spell to bring his father and dozens of other villains, most of them killed by Mordecai and Rigby, back from the afterlife.
The Carlocks (voiced by Mark Hamill, Roger Craig Smith, William Salyers and J. G. Quintel) are the villains that appeared in "Journey to the Bottom of the Crash Pit".
When Mordecai, Rigby, Muscle Man and Hi Five Ghost travel to their cave in search for a video camera, they find it in possession of the Carlocks, who refuse to give it back, forcing them to steal it an escape in a worn-out car.
The Zingos (voiced by William Salyers, Sam Marin and Armin Shimerman) are the trio of giant two-legged hot chicken wing creatures who live in the Inferno, and are the villains in "Bachelor Party!
After being forced to watch eight seasons of Huggstable cartoons while babysitting Starla's sister's daughter, Muscle Man had nightmares about them that lead him to constantly attack his friends while sleeping.
Benson then helps the park gang defeat the staff crew, and punches Doug, who falls inside the delivery truck, his head hitting the gas pedal, causing it to drive off of a hill and crash into a semi-truck, killing him for good.
Ygbir (voiced by William Salyers and later, vocal effects provided by Roger Craig Smith) Rigby's majordomo and later negative counterpart who appears in "Dizzy" and "Jinx".
He is an otter who is a serial scam artist whose strategy involves presenting himself as a drifter seeking work with the intention of taking over other people's lives, and is hired by Rigby as a temporary employee at the park to perform duties typically assigned to him by Benson.
Hurl Hurlbutt, Merle, and Burl (voiced by Roger Craig Smith, Matthew Yang King and Troy Baker respectively) are a trio of brawny surfers and the main antagonists in "Catching the Wave".
Snowballs (vocal effects by Mark Hamill) is a giant snow monster can breathe fire (despite the fact that it is made of ice) who appears in "Appreciation Day".
Rigby buys it to separate the bedroom, but after he and Mordecai abuse it, it gets too confused and begins to spread across the park, but the duo manage to catch it and hit the reset button, turning it off and returning it to normal.
He's made appearances in "My Mom", "Trucker Hall of Fame" (once before his father's funeral, and once during a flashback sequence), "Trailer Trashed", "The End of Muscle Man", and in "Terror Tales of the Park IV".
After finding out he was going to stay (due to Benson being unable to afford the new cart thanks to an accounting error), Mordecai and Rigby fished him out, repaired him, and drove him back.