Greg is one of the main characters in the episode "Alien Candy."
Greg is an alien disguised as a teenage human. He along with his acquaintance, Bonnie quietly attended Middle School looking for a human to eat. Greg helps set up 'Project Blue Bridge', a fake club for those who believe in extraterrestrials where he (and Bonnie) trick’s his victims Into eating a jellybean-like candy that helps him digest them.
During lunch, Greg overhears a football player named Dwayne bully a kid named Walt, because he was reading a book called The Quest for Alien Contact. Greg doesn't see Walt as a potential candidate for the "club" due to only being a sixth grader, but has second thoughts when he expresses his love for aliens.
He and Bonnie introduce themselves to Walt, who is slowly lured into the trap when told that if he joins, he would meet a real alien.
Eager to eat, Greg teleports to Walt's house one night with Bonnie asking him if he's been eating the candy. He is pleased when Walt says he did eat the candy and sticks a needle in his foot while paralyzed in order to get rid of some toxic blood. Greg confirms Walt is suitable for digestion and proceeds to take off his human costume revealing his alien form. However, when taking a bite out of Walt's toe Greg realized something was wrong and found that Walt is actually a redhead with dyed black hair. He reminds Bonnie that because their both allergic to redheads, they can't digest Walt, so they leave him be.
Greg knows that Walt will figure out he wasn't having a nightmare and will probably try to tell someone. Bonnie reminds him that no one will believe Walt if he tries to tell them. Greg then goes after Walt's best friend Tim.
He manages to trick Tim the same way he tricked Walt into joining Project Blue Bridge, except this time he tells Tim that Walt begged him and Bonnie to let him in the club and say's it recommended not to tell Walt about it (knowing Walt would he try and stop Tim from joining due to knowing about him and Bonnie being aliens).
Greg and Bonnie meet Tim later that night paralyzing him with their devices. As Greg gets ready to eat Tim, Walt comes in using an ax to break a pipe that sprays water on him, temporarily distracting him while he saves Tim. Greg chased Walt and Tim through the cafeteria, but he is killed when Walt realized that salt absorbs alien skin and sprays some on him.
Ironically, Greg's melted body is accidentally put in the school's homemade chicken nuggets, and he is eaten by the students.
- Greg is the first villain who was actually killed by the main character.