He first appears in the episode [I'm Your Biggest Fanatic." He appears after Kevin C. Cucumber tries to trick SpongeBob into falling into a hole with a queen jellyfish ship. SpongeBob and Kevin are shocked to see him in person, as they identified who he is. The king jellyfish takes notice of them and thinks that the ship is the real queen jellyfish and he chases it until he hits a billboard for Kevin's Ointment. It breaks and the king jellyfish realizes that it is a fake.
He then notices Kevin and gets mad at seeing him. The king jellyfish chases the Jellyspotters and SpongeBob into a cave. He notices that the group has gone into a cave; he puts on a football uniform and starts hitting the cave. Eventually, he stops and Kevin sends an anchovy, who, at first, says it is okay to leave, but then gets electrocuted and returns.
After the Jellyspotters give up, SpongeBob walks out. The king jellyfish gets ready to electrocute SpongeBob, who takes out a bottle of bubble soap and blows a bubble that looks like a pie. The king jellyfish cheerfully takes the bubble pie and leaves.
Battle for Bikini Bottom
In this game, Squidward gets electrocuted by some jellyfish and SpongeBob says that the only way to cure severe jellyfish wounds is to get some king jellyfish jelly. Squidward says that SpongeBob should scale Spork Mountain and fight the king jellyfish.
SpongeBob scales the mountain until he reaches the top, where he finds the king jellyfish singing in a giant shower. The king jellyfish is angry at SpongeBob for interrupting his shower. SpongeBob then fights him. SpongeBob wins and the king jellyfish gives SpongeBob a jar of his jelly and leaves.
The Yellow Avenger
He is in the Kelp Forest being attacked by three little Dirty Bubbles. The Yellow Avenger has to bounce on nearby plants to reach the king and then must throw waterballs in order to knock the bubbles off of him. After hitting him with three water-balls, he leaves and gives the Yellow Avenger access to the forest's clearing.
Battle for Bikini Bottom - Rehydrated
He will reappear in this game, reprising his role as the boss of Jellyfish Fields. He has also been redesigned, and is now slightly see-through like in "I'm Your Biggest Fanatic."
In Battle for Bikini Bottom and its remake, he is the boss battle in Jellyfish Fields on the top of Spork Mountain. The player, as SpongeBob, must defeat him to get his jelly to heal Squidward's wounds, which he got from jellyfish stings.
In the video game Battle for Bikini Bottom and its remake, SpongeBob accidentally disturbs the king jellyfish's shower during a request to obtain some jelly. He seems to have this anger in "I'm Your Biggest Fanatic." SpongeBob gives him a pie and they are thus on good terms.
In "I'm Your Biggest Fanatic," it appears that the king jellyfish is familiar with Kevin as he recognizes him and gets angry with him for something that he may have done to him. He then chases him, SpongeBob, and the other Jellyspotters.
King Jellyfish's Latin name is Cnidaria Rex. Cnidaria is the phylum of mostly aquatic marine invertebrates that have cnidocyte, a specialized stinging cell found in creatures such as jellyfish and sea anemone. Rex is Latin for the king.