Despite being one of the two main protagonists of the series, he also has done antagonistic acts which also makes him a protagonist villain, however he has done things to fix his mistakes along with Rigby (who is usually the one who causes it in the first place).
He is voiced by J.G. Quintel, the creator of Regular Show along with Hi Five Ghost.
Mordecai would be best described as mild-mannered and laid back and big hearted. Like Rigby, he lacks clothing, plays video games, and he drinks copious amounts of coffee and soda. Out of the duo, Mordecai displays far more patience, intelligence and responsibility than Rigby does. Even though almost every problem in every episode that was caused by the duo is Rigby's fault, Mordecai puts up with his laziness and immaturity, once filling out a job application for Rigby because he claimed it was "boring". Mordecai tends to be more sociable and friendly than Rigby, and has much more manageable relationships with others.
However, Mordecai can sometimes be a jerk to Rigby such as making fun of him for his shortcomings, repeatedly punching him when he was jinxed, replacing him with Skips as his teammate for the video game tournament, and making fun of his new name "Trash Boat", but despite bullying Rigby at times, he is a true friend. A great example would be in the episode "Death Punchies" where despite being dissappointed in Rigby for his actions, he admitted that he was wrong for mocking him along with Muscle Man and even let him be Player One after they made up.
Mordecai is also very forgiving as he continues to support Rigby despite his antics often getting them into trouble. He also befriended Benson and Muscle Man despite his differences with them. Also, unlike Rigby, he is mostly a devoted worker aside from getting easily distracted.
- He is sometimes a bully to Rigby and punches (it's usually for Rigby being a jerk or an idiot or both) or humiliates him for his amusement. There are also times where he takes his friend for granted; most notably during the later seasons. Furthermore, he often causes trouble with Rigby by either slacking or acting reckless. Despite this, Mordecai for the most part is a loyal friend and co-worker.
- The Power: After Rigby steals a keyboard from a wizard, he and Rigby start wishing for things which Skips was against. Some of the acts they committed included conning Pops and Benson, and several other things.
- Death Punchies: He made fun of Rigby's buttcheeks along with Muscle Man. His teasing of Rigby and not letting him be player one, pushed him into attacking everyone in the first place.
- Free Cake: He and Rigby lie to Benson about Skips wanting a surprise party and they (Rigby did this) interrupt Skips' spirit dance leading to his death by the Guardians of Eternal Youth, he and Rigby felt remorse but Skips got revived after giving them the cake for his life back.
- Grilled Cheese Deluxe: He and Rigby challenge each other on who is the expert liar and lie that they're astronauts and spend the whole episode lying until they revealed that they weren't astronauts. He was also likely going to hide the fact that Rigby ate Benson's sandwich from Benson.
- It's Time: He microwaves all the clocks in the house to prevent Rigby from finding out what time it is for his date with Margaret and kills him by pushing him off the microwave in anger when they get sucked in another dimension crossing the Moral Event Horizon. That said, it was an accident and he quickly fixed his mistake.
- Appreciation Day: He and Rigby write lies in the Book of Park Records to make themselves seem like better employees than they already are. This leads to the creation of Snowballs the Ice Monster and the near destruction of The Park.
- Jinx: He repeatedly kept punching Rigby when he was jinxed. He even punched Rigby for trying to get back at him after his actions result in a Jinx monster being summoned. Many people consider this to be his worst appearence during the eariler seasons.
- Do Me a Solid: He refused to do the last solid Rigby told him to do, nearly killing his friends by the destruction of the house until he finally gave in. That said, Rigby caused the situation to begin with.
- Karaoke Video: He and Rigby excitedly manage to accidentally trash talk Benson, Pops, and Skips. This lead to them ruining the Park's Karaoke Night as they got into a fight with the staff.
- Bet to Be Blonde: He kidnaps Rigby and tries to douse him in an extremely hot blonde vat but he was actually trying to get him to tell the truth that he cheated on the bet. While he was rightfully upset, his prank crossed the line and endangered Rigby's life.
- Video Game Wizards: He makes Skips instead of Rigby play with him in a video game tournament despite Rigby being his best friend but redeems himself in the end by making Rigby his teammate. He also showed little remorse until Skips called him out on it. On the other hand, he did it mostly because Rigby would have likely embarassed himself.
- Replaced: He along with Rigby, tried to sabotoge Jermey and Chad, so they wouldn't be fired. He also bragged about driving them away in the end (though this is karma for Benson trying to fire them).
- Best Burger in The World: Both he and Rigby killed their holograms for stealing their burgers. They also tried to disobey Benson several times.
- Trash Boat: He along with Benson, Pops, Skips, Muscle Man and High Five Ghost make fun of Rigby's new name "Trash Boat". While he did correct his mistake, he still went too far with his comments.
- Laundry Woes: He doesn't listen to Rigby trying to tell him to get over Margaret when they try to give Margaret her sweater and kicks Rigby out of the car when he calls him a "sad sack chump". He even left him behind despite Rigby having his best interests at heart.