Regular Show is about a kid named Chris. Chris loved cartoons, but soon got into Creepypasta. It started out with him reading Squidward's Suicide that a friend sent him to the link to, but soon grew into so much more. Before he knew it he had read every theory, lost episode, literally everything to do with Jeff The Killer or Slenderman.
Eventually he had grown a full scale addiction. It just started out as him trying to write Creepypasta articles, not doing very well but still trying. In time he started to purposely imagine Jeff The Killer standing behind him, or Slenderman hiding in every shadow and every tree, almost like he was preparing his brain for something insane to happen, and this made him jumpy, every noise behind him gave him shivers.
All of this was the safe part, after a while he was still seeing the faces, but now they weren't intentional, every now and then he'd see Jeff's face in a crowd of people, when he watched SpongeBob SquarePants his view of Squidward was permanently set to his face in Squidward's Suicide. His mind was screwed.
At some point his mother caught onto this, due to the fact that he was distant, jumpy and started talking to himself. So she took him to a doctor, who prescribed nothing more than some pills and some well needed sleep.
At first worked but it didn't take long to make his condition worse, he started to freak out more than when he could see ghostly faces every time he shut off the lights. On a really bad trip he ran away from home and collapsed in the woods about a mile away from his house. In his unconscious state, he had an insane dream in which everything was a cartoon,
Mordecai and Rigby were thought up because he saw a bird and a raccoon had ran past him just before he passed out. They both symbolize him in different ways, yet both are lay-around slackers. But with the fact that Mordecai always tries to get Rigby to stop his crazy idea, Mordecai is his conscience and Rigby is his side that supports all his crazy ideas.
Benson represents his father who would always try to make him do work, and try to get him to do more than sit around watching cartoons. The gumball machine represents a gumball machine his father gave him once for Christmas. The only real gift ever given from his dad.
Now I'm sure you'd expect Margaret to represent to symbolize a girl he liked and that liked him back but he couldn't get because he was never able to ask out because of his condition. Wrong. She actually represents his sister, not for the reasons that you'd think though, but because she was more successful, more outgoing, she even had a job at the local coffee shop. Chris idolized his sister and would do anything to impress her.
Muscle Man represents a friend (Mitch) that he had before he started losing his mind. This friend would always hit him, push him around, basically make him his bitch. The whole "my mom" thing is because Mitch would always make "your mom" jokes to literally everyone, but once got it wrong and said "my mom" and it became this whole ridicule towards him.
High Five Ghost was kinda like Mitch's side-kick. He would always high five Mitch after he made his "your mom" jokes, but would usually not say much. The reason High Five Ghost is a ghost, is because Mitch's side-kick, in real life, committed suicide because of the death of his father, which explains in the episode 'Trucker Hall of Fame', Benson tells Mordecai and Rigby that at Muscle Man's father's funeral, High Five Ghost "doesn't handle this sort of thing." This is in consideration of Muscle Man's father being dead.
Skips represents Chris's older brother. Much like his sister, he idolized his brother. Skips would regularly work out, to Chris, Skips was the strongest man ever, he also believed that Skips would never die.
Pops represents Chris's pedophile grandfather. That's why he has a somewhat 'cliché' pedophile, high pitch voice. He would play childhood games with Chris, which led to the unspeakable. In Chris's head, Pops was a lollipop for obvious yet sick reasons.