The rotten cupcake is an anthropomorphic rotten cupcake. He has muted pink icing, several sprinkles, and a rotten cherry on his head, the latter of which has been destroyed by Mowdown at least once. He wears a tan cupcake liner that has at least one pocket as his main form of clothing, which stretches over his arms and legs, over which he also wears black fingerless gloves and black boots.
The rotten cupcake has been shown to be less violent than the other students who are regularly in detention, most notably when he did not take part in trashing the detention room. He is not a skilled fighter either, as he is attacked and beaten by both Scythe and Mowdown on separate occasions, as seen in "The Lesson," despite Scythe being much smaller than him.
The rotten cupcake does not have a temper that can be easily affected, as he does not become angry when Julius breaks his phone in "The Line." In spite of this, he is capable of harming others severely, as seen in "The Uncle," when he attacks Gumball under orders from Julius. He will also do whatever it takes to be exempt from detention, even preparing to resort to extreme violence in "The Pact."
- "The Lesson": In his debut and first speaking role, he confirms that he is male. He is later attacked by Scythe, another student in detention, in both the Elmore Stadium and the detention classroom. Afterward, Mowdown rips off his cherry and causes it to explode.
- "The Finale": His cherry can be seen near Mowdown as part of the angry mob attacking the Wattersons.
- "The Uncle": His second speaking role, where he fights with and harms Gumball in the school gym until he is startled and tripped by Gumball accidentally, causing his head to be crushed.
- "The Weirdo": His third speaking role, where he takes part in bullying Sussie but is later blown away by Julius exploding in anger.
- "The Nuisance": His feet can be seen in the men's bathroom.
- "The Line": His fourth speaking role, where he is in line for the first screening of Stellar Odyssey, next to Julius. After they watch a spoiler-filled review of the movie, his phone is broken in anger by Julius.
- "The Pact": He, Julius, and Mowdown are sent after Gumball by Principal Brown, but mistakenly capture Darwin instead.
- "The Potion": A female in this episode, the rotten cupcake may have been kissed by Hector, which results in both her and her boyfriend, Scythe, beating him up.
- "Vote Gumball…and Anyone?": He runs for school president along with Julius and the other delinquents.
- The rotten cupcake is the only student who debuted in "The Lesson" to not have their name or nickname revealed.
- His voice in "The Weirdo" is different from his voice in "The Lesson," "The Uncle," and "The Line."
- The rotten cupcake told Gumball that he was male in "The Lesson." However, in "The Potion" Hector implies that he is a female who is the girlfriend of Scythe. This is likely a continuity error.