- “You'll never get rid of me! I'm part of you now! You can treat the symptoms, but you'll never cure the virus!”
- ―Monster Arm's last words
The arm is basically a large tentacle creature with a pink underside and light pink suction cups. Most of the arm is deep blue, and fades to purple at the tip of the tentacle, covered in pink slime. Its teeth are dull yellow and sharp, and it has a light pink tongue with spikes.
Role in the series
In its debut appearance, the Monster Arm is created by accident when Star tries to fix Marco's broken arm with her wand. The arm already shows it is beyond Marco's control, and begins making his social life difficult. After it beats Lars up and catching the attention of Jackie and other girls, Marco takes a liking to his new Monster Arm, and sees it as an advantage against his rival, Jeremy Birnbaum.
The evil within the Monster Arm would eventually begin to rub on Marco as he becomes very aggressive and hostile towards Star and his determination to beat Jeremy, and get Jackie Lynn Thomas to like him. Unknown to him, the Arm has other plans: to slaughter all humans and feast on the bowels. When Marco finds out about its intentions and finally sees it to be a bad influence on him, he gets Star to change it back to his normal arm. Before it is completely gone, the Monster Arm threatens that it would remain a part of Marco and would never be permanently gone.
- Despite his claim of returning, he never comes back after his debut appearance. Daron Nefcy revealed in a Reddit Q&A, that they did attempt to bring him back, but that they could not think of a way to do it. Though she has said that it is still in Marco and "might emerge one day".