The Jellyfish was introduced in the episode "Five Short Graybles." Princess Bubblegum placed the jellyfish inside of a machine to produce a tomato. Princess Bubblegum kept it on a plate covered by a little dome. Upon being uncovered, the jellyfish winced, moaned, and covered its eyes, possibly referring to jellyfishes living in darker areas of the ocean. Princess Bubblegum then combined it with a red balloon, thus producing a tomato that was sliced up by Peppermint Butler. The tomato went on the perfect sandwich, which was digested by Cinnamon Bun instantly.
The Jellyfish looks just like a regular jellyfish except its skin is opaque while most jellyfish have transparent skin. His whole body is a light-blue color.
This is the first character Princess Bubblegum has killed on-screen.