Although his best friend Daniel moves to a new neighborhood, Carlo is close enough that the two can still occasionally meet up. When Daniel and his sister, Kat, enter their new home, the two find a strange sponge-like creature living under their kitchen sink. The siblings fight over ownership of the creature, and Daniel is eager to show it off to Carlo. However, he is less than convinced by their story.
His mind soon changes, though as he sees for himself that the creature is alive. Immediately, he is intrigued, but his research is cut short by a nail through the foot. Before long, the creature seems to bring nothing but bad luck for the Merton family. Mishap after mishap seems to happen, and heir pet dog, Killer, goes missing. As the Merton siblings look for ways to destroy the creature, Carlo assists them, while Daniel searches for its true identity. Eventually, he finds it, in a book entitled Encyclopedia of the Weird. The book reveals that the creature is a Grool, a monster that feeds off the bad luck of others.
When all other attempts fail, Kat tries to kill the Grool by singing a soft melody to it. This ends up working, and the creature shrinks down until nothing else remains. As they celebrate their victory over the Grool, Killer comes back carrying something in his mouth. It's a potato-like creature with fangs, which matches the exact same description of the Lanx, the Grool's far deadlier cousin.
Together, Carlo and Daniel love to annoy Kat with their pranks. He shows a deep interest in the Grool.
The book describes Carlo as having shaggy black hair and brown eyes.
- Carlo's actor, Ashley Brown, voiced the character Bradley on the 1996 Canadian animated series Stickin' Around.